Perculas are not natural symbionts with BTA's, but they sometimes become host for Peculas in aquaria. The three natural hosts for Percula are: H. crispa, H. magnifica, and S. gigantea. A 20 gallon is a little on the small side for S. haddoni, they get large and eat a lot of food. A 30 gallon long is the smallest I would consider and a 40 breeder tank would be better. If you want to keep haddoni, then clarkii is a natural symbiont as is the sebae clown. If you are new to keeping anemones, you might find BTA's to be a good first choice. They tend to be forgiving in terms of husbandry demands. Also, BTA's host a wide variety of clownfish--including ocellaris and percula in aquaria (but not in nature).