Oddball said:
A 20's not big enough for the species of anemones that clowns form a symbiosis with. You have to treat the anemone the same as fish and a 8-10" diameter, 3-5" tall organism is just too large for a 20. Plus, they don't anchor and will "walk" around the tank looking for spots it likes. Chances are the clowns you buy were farm spawned and raised. These clowns have never seen an anemone and have not had the opportunity to build any immunity to the tentacle stings.
I understand the general logic you are basing this off of, and don't disagree that over time more space would be needed, but I do have some different opinions when it comes to the anemones, and in addition, clowns w/ anemones.
As far as keeping them in a 20 gal., as long as you stuck with one particular breed of anemone, or less agressive species (instead of choosing for example a Condi.. anemone and a bubble-tip to go together), I don't see why this would be a problem. I have had several anemones of all types, and no matter how much space I gave them (from 10 gallons to 500 gallons), they are just gonna go where they want to go, usually anchoring their foot in a small crevice. I have seen them jam themselfs up between rock structures and the glass -- looked really un-comfy, so I moved him. Apparently it wasn't as the next morning, it was in the exact same place I had removed it from, in full "bloom", and thriving. Certain species of anemones should not be kept in close proximity of one another, as they can be agressive such as a Telia (a.k.a strawberry) anemone. To ensure you do not unknowingly induce a chemical warfare, or more often, physical battles, make sure your anemones are fairly compatable (mainly, just the lessor agressive types), and that there is plenty of space for them to live without crowding each other. For a 20 gallon, I wouldn't suggest more than one or two, and get them small.
As far as captive bred clowns not being able to stand the stings of an anemone -- I don't believe this to be true. Anemones are a major source of protection and shelter from the surrounding waters for a percula clown, and other types of anemone fish. The clown's immunity to the potentially harmfull stings from a host anemone, are derived from the thick mucous coating it secreats. No matter whether the fish was WC (wild caught) or CB (captive bred), the symbiotic relationship between these two animals is instinct. In an aquarium, when a new anemone is introduced, typically in the beginning the clown will display similar behaviors of actually being stung. However, the fish will return to the anemone time after time, and gradually allow the tips of the tetacles to brush against its fins, and then to the rest of its body, elaboratly swimming about darting in and out of the anemone. During this "dance", it seems as if not only is the mucous coating of the fish spread onto the anemone, but that the mucous from the anemone is in return, spread onto the fish. Both animals' mucous secretions contain chemical compunds not too different from one another, therefore, neither views the other as an enemy/food. They live peacfully together, and have a give-and-take relationship; as the anemone provides shelter, and the clown can be a "cleaning lady" of sorts, removing debri, and bits of uneaten food from the tentacles of the anemone. Not to mention that since clowns become very territorial of their anemone homes, they in turn protect the anemone from certain would-be predators.
It's up to you as of what you decide to do, but don't look at it as impossible because you are starting off with a smaller tank. Just know that this won't be sufficient for long, and if you decide to keep some of the larger species, you will need a lot more room. For now though, buy them little, and in small numbers to ensure you do not over-crowd (by this, i mean not giving agressive animals room of their own, not "over crowd" as in volume of live stock - but don't do that either, lol).
--EMI
