+1 with Miz. For a 20 gallon you are pretty much stuck with lps as a host. As a rule an anemone that is to host clownfish must be at least 1.5 to 2 times bigger in diameter than the length of the largest hosting clown so that the fish don't kill the anemone. Clownfish will host quite a bit if different corals. Long tentacle plates, mushrooms, toadstools, acans, sinularia, zooanthids, brain corals, any of the frogspawn family, and I've even seen a clown hosting an elegant coral.

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