FAQ about keeping Elysia (with focus on E. clarki):
Are they hard to keep? Depends on your point of view. Their needs are simple and specific: correct food plant, no strong pumps, relatively stable temperature and salinity. If you can keep their food plants alive, you can keep Elysia. Because you don’t need to add food, and the plants consume nitrate and phosphate, fancy filtration and protein skimming are not only unnecessary but probably undesirable. You may need to supplement nitrogen and phosphorous, as well as calcium and carbonate, for good plant growth.
I have a problem with Bryopsis. Can I put one in my reef tank? Pretty emphatic “no” on that one. The strong circulation and open pump intakes in a reef tank are incompatible with the survival of Elysia. Although they are routinely offered for sale and added to reef tanks, to my knowledge, no Elysia has survived for very long. On the other hand, keeping a small group in an in-line refugium with low flow and appropriate macroalgae would most likely be quite successful.
Can I use them to clean the algae off my glass? No. They feed on filamentous algae, rather than the turfs or diatoms that obscure your view through the glass. If, on the other hand, you have a tank that fits their needs (see above) and has an infestation of hair algae, they may just be the slugs for you.
You say they are photosynthetic. Do I really need to provide food for them? Yes. Although photosynthesis can provide some energy to the slugs, they will starve and ultimately die without food. This has been demonstrated in the scientific literature (e.g., Crista et al., 2013), and is consistent with their ravenous appetites even when provided with intense lighting.
Can they eat prepared foods? No. They are specialized to suck sap from their food plants, so flakes, pellets and the like will be ignored.
Will they breed in aquaria? Emphatic yes. If they are fed well and you have two or more, breeding seems to be a routine occurrence. They lay coiled egg masses with some regularity. The eggs hatch into veligers (the planktonic phase) that can live off their yolk for the few days before they metamorphose into little slugs. There are numerous reports of them having reproduced successfully in hobbyists’ aquaria (one is described here), as well as in academic settings (see the Aquaculture page).
What do I feed them? Depends on the species. Most species of Elysia will eat more than one species of algae, but each has its own favorites. My personal experience is with E. clarki, which will eat Avrainvillea, Bryopsis, Derbesia, Halimeda, and Penicillus, although Bryopsis is their clear favorite. Halimeda and Penicillus are generally available online (see Resources), and you can probably obtain Bryopsis free of charge from a local marine aquarist who has an infestation and thinks you are an idiot for wanting the stuff.
Can I keep other organisms with them? Certainly. Given their diet, the slugs are unlikely to cause harm to even the smallest and tastiest animals. Corals that can tolerate low flow will thrive, and fish that have similar requirements, such as seahorses, are excellent tankmates. Conversely, because chemicals from their food plants make them less palatable, they are relatively safe with most fish. Naturally, mixing Elysia with large, aggressive, voracious predators should be done with caution.
How can I build my own Box of Slugs? This particular cat can be skinned in any number of ways. You will need a bare minimum collection of equipment:
Tank: 10 to 20 gallon (40 to 80 liters) will work just fine. For better light penetration, shallower tends to be better. A glass lid will help reduce evaporation.
Filtration: Not really. Growing plants will consume any wastes that are produced.
Circulation: Gentle flow, from a small powerhead or airstone. If using a powerhead or other pump, screen the intake with fine mesh, like a filter bag, to keep slugs from being pureed, and clean the screen from time to time. Airstones avoid this problem, but the fizz will generate salt creep and increase evaporation.
Heating: Tropical and subtropical species (e.g., clarki, crispata, ornata) will generally need the tank to be heated to 76-ish degrees F (25ish C), while temperate species (e.g., chlorotica) will probably require chillers.
One solution for both heating and gentle circulation is the Hydor Ekip thermopump. Unfortunately, it has been discontinued.
Lighting: Success can be had with LED, Metal Halide (MH) or even Compact Fluorescent (CF) lighting, assuming there is enough power. 100 watts of CF light works well over a small tank, and 70 or so watts of LED or MH will do the trick.
Livestock: In North America, the easiest slugs to obtain will be E. clarki, which can be easily maintained on Avrainvillea, Bryopsis, Halimeda (not preferred), and/or Penicillus. See Resource page for sources.
Maintenance: Not a lot, but you may need to supplement nitrate and/or phosphate in a fishless system. Calcareous algae, such as Halimeda, may also require supplementation of calcium and carbonate.