Article: Planted Turtle Tanks

By Richard Lunsford, Kent Hunsuckle & Rone Fong.

At some point many turtle hobbyists ask about live plants; can they exist in a turtle tank, what kind work and what are their needs? Experienced hobbyists give a range of answers but often limited information; many people very knowledgeable about turtles have never researched & maintained planted tanks, and some never branched into the fish aquarist hobby far, either. Some recommend, hey, sure, get some live plants; they look better than plastic plants, provide theoretically needed stimulation via a detailed environment and draw wastes out of the water. Some say ‘no,’ your turtles will inevitably either eat or dig up and kill any plant. So who’s right?

I undertook this article with limited live plant maintenance experience, acquired through research and personal experimentation. Planted tanks are commonly discussed amongst fish enthusiasts, and many people actually have ‘planted tanks’ with very few & very small fish whose purpose is to produce plant fertilizer; the plants aren’t supporting cast, they’re the main event. That won’t be the case in a turtle tank.

To start with, yes, you can have live plants in most turtle tanks. That’s a qualified yes. It varies with tank size, substrate, lighting, water parameters, CO₂, plant number & species, turtles’ number, size, species and individual variation, and other tank inhabitants.

Let’s take a look at plants’ general requirements; substrate, atmosphere, light & nutrients. Then we’ll look at what plants can do for you, and what kinds you might like.

Note: This Article relies on hyperlinks to present a lot of info. concisely, so you can ‘follow through’ via hyperlinks to learn more where you need to, without having to slog through reams of other info. Click on plant names to summon sites with species-specific info. If you right-click the mouse you can ‘Open In New Window’ & open hyperlinks without losing sight of this article.

I.) What Plants Require.

1.) Substrate – In nature you seldom find a pure gravel bed in a water body, yet many aquarists expect plants to thrive in 2 inches of aquarium gravel. People with ‘plants only or mostly’ tanks have some soil-mix options, but turtles will turn any ‘dirt substrate’ into a muddy slurry. So what can you do?

a.) Use Plant Species That Aren’t Root-Dependent – many plants (i.e.: hornwort & Anacharis) draw nutrients out of the water column; Anacharis will send out root-like growths to attach itself to substrate, but can thrive free-floating in the tank. Hornwort will end up floating whatever you do. Floating plants require nutrients just as root feeders do, but do not require a nutrient-rich substrate.

b.) Provide a Nutritious Substrate – There are two popular products widely recommended.

i. Laterite is a fine reddish clay gravel that provides an iron-rich substrate in your tank. Cheaper than Flourite but reputedly not as ‘nutritious.’ Often mixed with gravel for iron and to improve growth. Rinse thoroughly before use or have cloudy red water for days. As of 1-19-03, Pondside Herp. Supply charges $12.00 for a 55 oz. box, sufficient to mix into the gravel of a 55 gallon aquarium. Read another article on Laterite.

ii. Flourite is a mildly abrasive hard reddish clay gravel, one of the best out-of-the-box substrates you can buy. It does not crumble to mud in water, but gives off a lot of reddish ‘mist’ in the tank when first used. Thorough rinsing helps but won’t eliminate the problem, which fades with time. Flourite is used as pure substrate (best) or mixed with gravel (cheaper). Due its mild abrasiveness I don’t recommend it for softshell turtles, which try to bury themselves in substrates. As of 1-19-03, Pondside Herp. Supplies charges $12.50 for a 7 kg bag, said to provide about a 2 inch deep layer in a 10 gallon aquarium. Recommended use is 1 kg for about each 200 cm² (31 in²) of tank bottom. (Example: A 75 gallon tank is 18”x48”, 864 in², roughly 28 kg Flourite recommended, about 4 bags. Not bad…)

c.) Pure Aquarium Gravel – some plants can survive in this. Your best candidates include those that don’t ‘really’ root (like Anacharis), a few particularly undemanding plants (Aponogeton ulvaceus), or plants that aren’t put in substrate (Java fern & Java moss). Kent recommends sticking to ‘natural’ gravels (not painted/colored gravel, where the coating will be worn down.) Kent found those with a high quartz content seemed to adversely impact pH, but quartz is inert; I suspect some gravels release minerals into water & add buffering, whereas quartz wouldn’t). Usually medium or small gravel is used (in which case watch that your turtle doesn’t swallow it & risk impaction). Kent’s had the best luck with regular small gravel straight out of the bag with all the dirt in it (not rinsed), & recommends 3” depth for plants with large, sturdy roots (like Anubias barteri).

d.) Sand – Kent stated it’s not the best substrate for plants; it lacks most nutrients, packs around the roots too tightly & doesn’t allow for water flow around the roots to bring in nutrients. Kent said it can be used when Laterite is mixed into the sand, but recommes only grassy plants or some slow-growing plants that slowly grow strong roots be tried in sand.

2.) Atmosphere – Relevant concerns are submersed/emersed plants & CO₂ supplementation.

a.) Submersed/Emersed: Assume any aquatic plants at the pet store are designed for permanent submersion and you assume wrong. Some plants are indeed fully aquatic, or at least can exist indefinitely submerged. Others, such as terrestrial plants or some emersed plants (i.e.: Green Acorus Grass), by nature grow in water but extend much of the plant body above water. Such plants may die a slow death if kept permanently submersed. On the other hand, they may be perfect for a spotted turtle or Eastern mud turtle tank with fairly shallow water.

b.) CO₂ Supplementation: Most turtle tanks are sparsely planted & this is a non-issue. The CO₂ gas level is a common growth-limiting factor in planted tanks. CO₂ is produced from respiration by animals (and even plants), breakdown of organic materials and diffusion of atmospheric CO₂ into the water. Since turtles exhale above water (unlike fish), they don’t enrich tank CO₂ as much. You can raise CO₂ levels via CO₂ supplementation, and there are many systems available. Some are fairly simple & rely on a yeast/sugar mix to produce CO₂ bubbled into your tank via plastic tubing, sometimes into filter intakes to make water absorb it (rather than let it bubble straight up & out). Some commercial systems use CO₂ tanks you pay to have re-filled (perhaps annually). Some use carbon blocks. Commercial products may only produce CO₂ during daylight hours (when plants use it), monitor tank pH, keep a constant bubble rate, etc… Even systems rated for tanks smaller than the one they’re on often produce much quicker growth. If you overdo it, large pH shifts can occur & kill your fish. This is more troublesome at night, when plants don’t consume CO₂. DIY (Do-It-Yourself) systems are cheap & design plans available online; you have to mix the yeast sugar mix & change it every few weeks (Rone found this article on making a DIY injector). Rone also cautioned that you should read through 'Debates and Miscellany' on the bottom of the article and/or 'Take Heed: CO2 is not a Toy'. Commercial systems are precise, reliable (no yeast mixture to die off suddenly), low maintenance & offer more control. Filters whose output causes a lot of surface turbulence work against supplementation because higher CO₂ levels are lost faster from highly aerated water. Rone found an alternative, Seachem’s Flourish Excel, which provides carbon in the form of simple organic compounds rather than CO₂ gas. Most turtle tanks are sparsely or at most moderately planted, so competition for CO₂ is probably light (though there are fewer fish producing it). Bottom line: You can get away without supplementation, but it’s there if you want fast growth or have trouble with a particular species. You can try a DIY system to see how it works, then buy a commercial system once you decide on permanent supplementation. Or give Seachem’s Flourish Excel a try (the simplest way to experiment).

3.) Light – The usual recommendation for planted tank lighting is about 2-3 watts/gallon. The aquarium hood lights most tank setups came with produce maybe 1 watt/gallon. This leaves you two choices: either use low-light tolerant plants or provide more light.

a.) Low-Light Tolerant Plants – often grow better under stronger light, but can thrive in low light. Examples include Java Fern, Java Moss, Anubias species and some Cryptocornye.

b.) Provide More Light – That means buying a new lighting system using more powerful technologies, which are quite expensive and the bulbs should be replaced periodically. Note that the relatively low-wattage so-called ‘plant bulbs’ commonly sold for regular fixtures in pet stores are no substitute for an increase in the amount of total light. There are a number of options, including VHO (Very High Output), Power Compact Flourescents and Metal Halide. I recommend Power Compact Flourescents for most users.

I.) Power Compact Flourescent Fixtures – use a long, very narrow fluorescent tube folded back on itself. These bulbs are too high-wattage for standard fixtures & require Power Compact Fixtures or a retrofitted standard light fixture. Produce a lot of light, and little heat. Lighting is rather even. Common bulb strengths are 48 watt, 65 watt & 96 watt bulbs. (Note: Some fixtures are designed to hold 2 bulbs so you can add a ‘blue actinic’ bulb, a feature designed to provide specialty lighting to marine & reef tanks). For an example, check out this Compact Sea Life Britelite model, a 4 foot fixture with 2 65-watt bulbs for ~ $150.

II.) Metal Halide Lighting - Bulbs resemble incandescent bulbs, not fluorescent, and produce a lot of light and heat. Lighting isn’t even, an effect some like & some don’t. The bulb is shaped more like an incandescent, not long like a fluorescent.

III.) Natural Sunlight – There’s nothing like it for intense full-spectrum ‘everything you need’ lighting. Plants love it. There’s also nothing like it for rapidly overheating the tank & killing your turtles, and for triggering massive green water algae blooms. I don’t recommend putting a tank near a window; UV light doesn’t penetrate glass anyway & the risks outweigh the benefits. Outdoors, be sure you use a large water volume, always with partial shade, and you may still battle green water algae. Note: I have accidentally killed turtles via overheating & it is horrible.

4.) Nutrients – Plants require nutrients, especially nitrogen and phosphorous. In most tanks phosphorous is abundant & a non-issue (due to commercial food introduced into the tank, etc… Reptile hobbyists can tell you it’s usually a battle getting enough calcium into the diet to balance their pets’ phosphate intake, not getting enough phosphorous into the food). Your cycled filter breaks pet waste down; ammonia to nitrite to nitrate, so nitrogen compounds are abundant. Potassium and iron are other nutrients noted to be important. You may not require nutrient supplementation for hardier species, especially if your substrate includes Laterite or Flourite, but supplementation is available. Seachem’s Flourish is a reputable reasonably-priced product sold at PetsMart (used weekly; I think my bottle was ~ $8 & should last a long time). Rone noted competing product Yamato Green. Kent said while not critical in well-stocked fish tanks nutritional supplementation is important in turtle tanks given the amount of cleaning these tanks receive. This is especially true for bare-bottomed tanks where turtles are fed outside the tank. Rone advised watching the pH if you add much fertilizer, and remember it may contribute to algae blooms. She found this Krib Article on Nutritional Deficiency to review if your plants don’t do well.

II.) What Plants Can Do For You.

Plants offer a number of advantages to the aquarist.

1.) Beauty – Even the best plastic & silk artificial plants do not match the realistic beauty of live plants, a fact more evident after you’ve worked with both.

2.) Environmental Enrichment – It’s thought by some turtles may enjoy a varied enclosure with places to explore & search for food, rather than just a glass box of clear water with a basking log & 2 inches gravel substrate. Live plants are a part of that, along with driftwood, synthetic logs & rocks, etc…

3.) Naturalistic Enclosure – tanks with live plants look more like a ‘slice of nature’ than a glass box of water.

4.) Cover for Prey – live food can evade the turtles better with cover. This extends the hunt. In outdoor enclosures, this cover may allow goldfish to breed.

5.) Waste Removal – nitrogenous wastes are removed by plants. They will not keep your nitrates at zero, but a well-planted tank may run in the 20-30 ppm when you’d have expected double that. Someone posted of a box turtle getting repeated ear abscesses until a filter was installed on its water bowl per Vet instruction; I suspect a clump of Anacharis might have similar benefits to filtering.

6.) Out-Compete Algae – live plants draw nutrients needed by algae from the water column, which can reduce algae growth.

7.) Support – In outdoor enclosures some plants, like water hyacinth, offer resting places for small turtles.

8.) Enjoyment of Keeping – It’s more fun to work with a variety of living things, be they turtles, fish, snails or plants. Live plants are satisfying to grow.

III.) Plant Species of Interest.

Let’s take a look at plant species grown in turtle tanks.

1.) Anacharis (a.k.a. Egeria densa) – an older name for a plant with a newer scientific name, but adding to the confusion Anacharis is used more broadly for different related species. And Elodea is sometimes used instead of Egeria! The two are not the same. I think what you buy at PetsMart is Egeria densa. What you should see in native U.S. water bodies is the very similar-looking Elodea Canadensis or nuttallii, but Egeria densa is an invasive plant in the U.S. (see map). To clear up the confusion read this article. Usually considered a cooler water plant that grows darker & thicker that way (at PetsMart usually dark green & thick), but lightens & thins out even in well-lit & nourished tanks. Fair low-light performance, but gets even lighter & thinner. Anacharis is not a rooting plant but sends out root-like projections to anchor itself, & can grow floating or with the base of several stalks banded together & stuck in the substrate. Capable of splitting off bicarbonate ions from water if CO₂ levels are low (makes its own CO₂), which can lower tank pH. Grows fairly fast under good conditions & consumes some nitrates & nutrients. Said to produce a substance to discourage blue-green algae growth. Herbivorous semi-aquatics such as sliders, cooters & painted turtles love Anacharis & make a big mess if given a lot at once. It cannot be grown in their tanks. It can be grown in tanks with mud/musk turtles. You may be able to harvest it from local water bodies, but run a risk of introducing parasites into your enclosures.

2.) Java Fern (Microsorium pteropus) – A hardy plant grown submersed or emersed, low-light tolerant, slow-growing with tough leaves & usually not eaten by turtles. Java fern is neither planted in substrate nor left free-floating; the root is tied to driftwood (or other rough objects like rocks) with thread, & develops its own attachments over time. Kent noted you can wedge the roots into a crack or hole instead of tying it in place. This makes it a good ‘portable’ plant. Driftwood with screws or nails & suction cups can be stuck anywhere on the tank wall with Java moss attached. Juvenile plants may grow from its leaves. May come to look rather bedraggled in a turtle tank, but hardy. It’s from Southern China, India and the Philippines.

3.) Java Moss (Vesicularia dubyana) – Hardy, low-light tolerant & turtles usually don’t eat it. Looks like moss. No relation to Java Fern, but likewise is tied to a rough object (driftwood, etc…) with thread & attaches itself over time. Kent said tie loosely or the parts tied off may die. Rone noted you can leave it free-floating. Tends to spread, for which reason some eventually get rid of it. Some have successfully turned it into a ‘lawn’ across the substrate, but this will make gravel vacuuming impossible & isn’t recommended. Can entangle other plants. A similar plant marketed for cooler water tanks is Willow Moss (Leptodictyum riparium.)

4.) Hornwort (Ceratophyllum demersum) – A very fine-leafed branching stem plant growing in floating mats very similar to milfoil. Fairly low-light tolerant but prospers under more light. Very fast grower (much faster than Anacharis) & removes wastes from water, but small branches and tiny leaflets are brittle & break off, cluttering the tank so you will eventually get rid of it. Several stems can be tied together at the base & stuck in substrate, but it will free itself over time and float. Unchecked hornwort will give your tank a Sargasso Sea effect. Turtles may eat some but not with gusto & it might prosper even in tanks with sliders, cooters & painteds. Supposedly one of few plants that can survive in a goldfish pond. A very similar invasive (non-native U.S.) plant in my area is milfoil.

5.) Red Ludwigia (Ludwigia repens) – A mid-sized native U.S. stem plant planted in substrate with green stems & leaves some of which turn a reddish light tea color. Turtles don’t eat it. Quite beautiful, hardy, reproduce fairly quickly, young plants growing off the parent stalk give a branched look, and growth rate is moderate. They are planted in substrate. They are not low-light tolerant, but moderate light (around 2 watts/gallon) will do. Turtles easily uproot them digging, but they tolerate replanting very well, even if left floating around a couple of days first. A good choice for large tanks with few turtles of small species (mud, musk, Southern & midland painted and Cagles or Texas maps).

6.) Duckweed (Lemna minor) – A tiny floating native U.S. plant growing in dense thin mats atop still water. Due to rapid growth & nitrogenous waste removal it’s used in some areas for sewage water treatment and the plant itself is fairly high protein. In tanks with no herbivores it tends to form a green mat over the tank water, darkening the water and preventing UV-B light from getting into those first few inches of water for your non-basking turtles. In tanks with herbivores, it tends to disappear (they eat it). In shallower tanks it’s prone to coat the filter intake and may be a nuisance. Kent recommends avoiding duckweed in tanks with ‘water fall’ filter return (i.e.: AquaClears) since the waterfall drives duckweed underwater where the intake grabs it. The Charms of Duckweed provides a lot of info. on the plant & its many roles.

7.) Water Hyacinth – A floating plant native to South America where it grows in dense, thick mats (up to 2 feet deep). Illegal in some parts of the U.S. (like Texas) and cleanup is ongoing & extremely costly in some others (like Florida). Grows rapidly & draws considerable waste from the water. The feathery black roots are preferred resting sites for young mud/musk in larger tanks. Herbivorous turtles often eat them. Unfortunately, I had lousy luck growing them in a 200 gallon aquarium with nearly 400 watts total lighting, bits of the root system broke off and made a mess, and I finally tossed them out of the tank. A similar but much smaller plant I had the same experience with was Water Lettuce. Don’t introduce water hyacinth into your natural waterways; the thick mat out-competes native species, casts the water body into perpetual night, releases oxygen into the air so the water can become oxygen deficient & produces large amounts of waste onto the bottom (massive amounts of rotting dead hyacinths). Check this map to see if they’re in your area & read this article on invasive issue. One forum regular keeps water hyacinth outdoors in separate containers & adds some to her RES tanks to graze on, changing plants to keep them alive. Another approach for outdoor ponds is to have a separate turtle-free pond section that water’s pumped through so water hyacinth can remove wastes. Note: no relation to the land-based Hyacinths, which are poisonous to turtles.

8.) Amazon Sword Plants – Many species, small to large, reputedly bitter-tasting and many are hardy. Tend to require strong light and are not low-light tolerant. Medium & larger species are often used at the back of planted tanks. They are planted in the substrate. My Southern Painted didn’t eat much but bit the leaf stalks off & killed it; I saved a small melon sword from a similar fate. Feel free to try them but don’t get your hopes up. Kent said Pleco.s eat sword plants. Note: the famous Micro Sword is a dead-ringer for lawn grass & a much sought after plant for an aquatic ‘lawn.’ Unfortunately, it requires strong light & generally won’t prosper in a plain gravel substrate; I doubt it’d tolerate constant trampling. I’ve never seen a turtle eat it. A very large tank with one or two small turtles & a Flourite substrate might let you use this.

9.) Anubias species – Comprised of many varied-formed species, Anubias are famous for being tough, inedible, low-light tolerant slow-growing plants that are hardy & do well. Because they are slow-growing the algae may over-take & cover the leaves, which is complained of occasionally. They may be slow to re-grow leaves the turtles knock off. A good choice. Some common varieties include A. barteri v. nana (tie to a tank fixture with black thread; if you must plant in substrate, don’t completely cover the rhizome as it may rot) and A. barteri coffeefolia (can also be tied to driftwood & stones).

10.) Cryptocornye species – A large group of fairly low-light tolerant plants of varying hardiness and size known for their dislike of being uprooted & replanted. Some common species are fairly hardy & can prosper in large, sparsely populated tanks. Turtles do not eat them. I expect they would work best with smaller turtles in large enclosures (trampling is an issue). One of the most well-known species is the wendtii, available in both brown & green varieties.

11.) Aponogeton species (ulvaceus, crispus, etc…) – Popularized by the A. ulvaceus bulbs sold in Wal-mart’s pet section. Fairly hardy & ulvaceus may be somewhat low-light tolerant. These plants produce long, broad leaves from a central root/bulb in the substrate. Turtles usually don’t eat them but may ‘sample’ the leaves. The leaves are moderately durable but in turtle tanks get somewhat ‘damaged.’ They may grow in plain gravel substrate. These plants are accustomed to a dormancy period and so may someday seem to die down inexplicably; I’m not up on how to manage that. Some people just replace them.

12.) Green algae (fixed, not free-floating) – Contrary to what you’ve been taught, algae is not dirty or filthy. It’s simply a unicellular plant. As such, it draws nitrogenous wastes & nutrients from the water, & light drives photosynthesis to make sugars & give off oxygen as a by-product. It makes synthetic tank fixtures & driftwood look more natural. Some turtles may gnaw at tank furnishings trying to eat it. I recommend you clean your tank’s front glass plate off with an algae magnet (I recommend the Mag Float) & let the rest be, or use algae eating fish (plecostomus, Chinese algae eaters, in some cases Otocinclus). (Note: Pleco.s are too large & heavy to clean small leaves; snails are second-rate algae eaters). Kent said snails eat some algae but some species eat plants, too; I’ve never been impressed with snails as algae eaters. It’s a natural part of the tank’s ecosystem & not to be eliminated. You can reduce it by reducing the nutrient load in the water (feed turtles outside the tank, step up water changes, etc…) or reducing the daily time lights are on.

13.) Filamentous (‘Hair’) algae – Similar to fixed algae but has a longer, green shag look. May grow over your regular plants. Some turtles graze on this. Fish willing to eat it include American Flag Fish (Jordanella floridae), Rosy Barbs, Siamese algae eaters, guppies, sword tails & mollies. Be warned guppies breed like rats.

14.) Brown algae – Actually diatoms, which form a brownish haze over your gravel & prosper in lower-light conditions. Unlikely to be an issue. Otocinclus are said to eat it.

15.) Blue-Green algae – Now considered more of a bacteria (cyanobacteria) than a plant, but photosynthetic none-the-less. Tends to look like a dank green slime mat on gravel and tank fixtures. It’s hard to find anything that’ll eat it (although I found an article from someone who seems to think you should; you’re on your own if you try this…). Can be killed with Erythromycin sold for fish diseases but likely to come back. Reportedly eaten by the Butterfly Goodeid (Ameca splendens) – a hard-to-find platy-sized fish. My best advice is to suck up what you can with a gravel vacuum during water changes & hope your plants out-compete it for nutrients over time.

IV.) Which Turtles are most Plant Compatible?

The best matches for a planted tank are the mud/musk, which stay small but may dig up plants, and the more carnivorous and smaller male map turtles. Southern & Midland painted turtles are small enough to co-exist with plants in larger enclosures, if those plants are inedible. Sliders, Cooters & larger painteds may co-exist with the toughest plants (Java moss), but will tear the more leafy varieties (Aponogeton) & constantly uproot the poorly rooted (Red Ludwigia). Softshells may dig up plants while burying themselves. Common snappers are powerful & destructive and may be incompatible with most plants, but driftwood with Java moss should prosper, or perhaps Willow moss in cooler outdoor enclosures. Anacharis or duckweed may grow in a snapper enclosure, but snappers may eat them.

Kent has recommendations for a happy marriage of plant and turtle; for softshells, plant along the edges and corners to reduce uprooting. To prevent uprooting generally, place heavy items like driftwood or smooth rocks around the plant base.

V.) Useful Online Sources of Plant Information.

1.) Tropica Aquarium Plants Index & Info. – the place to get the low-down on individual plant species. Click on a name, get a summary on it.

2.) The Krib Plants Section – an older site whose information sometimes has a UseNet style but includes great discussions on matters such as substrate, lighting and fighting blue-green algae. Almost anything you need to know about general planted tank care is covered.

3.) Fish Profiles – a top-of-the-line fish forum that includes a plant forum. Great place to ask questions & learn more. Membership is free.

4.) Aqua Botanic – Online collection of many plant-related articles. Their Beginners’ Page is worthwhile. They have a gallery of planted tank photo’s.

5.) Robyn’s Algae Page – part of the excellent ‘Robyn’s’ series covering a range of aquarium topics. This page discusses the types of algae & compares algae eaters in an extensive table. Links to specialty pages for varied algae eaters are quite valuable. You want this one bookmarked!

6.) The Krib’s Algae Section.

7.) True Aquarium Plants’ Lighting Article – a decent primer on lighting.

8.) The Krib’s Article on DIY CO₂ Injectors.

9.) The Krib’s Article on Nutritional Deficiency (actually an extended discussion).

VI.) Places to Buy Plants Online.

1.) Arizona Aquatic Gardens (Read their ordering guidelines; one thing they state is “Plant Orders must total no less than $35.00. This includes snails and other living things.” Well-known & reputable.

2.) Florida Driftwood (also sell American Flag Fish). I’ve bought from them.

3.) True Aquarium Plants (I haven’t used them yet).

VII.) Places to Buy Flourite & Laterite Online.

1.) Pondside Herp Supply.

2.) Arizona Aquatic Gardens.

VIII.) More Information on Lighting.

1.) Custom Sealife’s Page on product options for varied sized tanks.

2.) The Krib Lighting Section.

IX.) Information on Plant Species of Interest.

1.) Anacharis.

a.) Article breaking down types & naming.

b.) An article from the Robyn’s series on hardy submerged plants.