I have to go with the majority... You can get away with a lot when fish are small. A 30g will work for a pair of convicts.. OR.. a female salvini (I'm not sure if a male would be comfortable). A pair of firemouth would just about be the max for that species (assuming this is a 36" tank). Redtailed sharks get nasty as they get bigger. Your brother needs to be prepared to separate his fish soon (that convict pair isn't going to wait long to spawn), or he's going to lose some nice fish. Which dithers to add is the LEAST of his problems!
