maybe try 3 or 4 Hoplo cat fish they clean the bottom of a tank of any left over food spotless and if you dont feed them for a day they will even clean up the poop from other fish!
i keep them with severums and they use to be with an oscar before that, they grow to 4 -7" but are quite thick and really thick armour i doubt much would bother them unless something could swallow them whole including their pectoral fins which have a very thick bone structure.
Bold little cats as well, there out night and day constantly moving and will try cleaning other fish like severums and oscars if there is no food about.
