i would suggest a pictus catfish, they're so awesome! i had one for three years, and they are so active. they are fun to watch, and if put with the right fish, are very social and friendly. they get along very well with larger fish. but smaller fish tend to disappear quick, so leave out tetras, barbs, danios, exct. but be careful when netting them, because they're dorsal fin has a hard, sharp spike on it that will easily tear up a net. plus when they get bigger, they can stick you with it. also, they have very sensitive skin, rather than scales, and are EXTREMLY sensitive to medications, so be careful when treating them, or the tank for things like ich (that's how i lost mine). if your treating the tank, remove the pictus. and if you're treating the pictus, try using a salt bath. also, if your getting more than one, make sure they have enough room to establish territories ( your 55g should be fine for several, because they only get 6" long) other than that, they're great fish to have! enjoy the wonderful hobby of fish-keeping! ( i, personally, am in love with it! ) :thm: