You want to add 50 more individuals to this 55??? I would say 15-20 max, and I would do a bit research on how to properly pick out stock. I know it's easy to just buy from assorted tanks, but the problem is that many species do much better in groups with a m:f ratio of 1:3-4, and many species are actually not compatible. Rarely will johanni, auratus, Kenyi, crabro, etc do well with the more relaxed species like yellow labs, rusties, zebras, etc. I would plan out a stock list and then buy based on that, it will save you many headaches when the fish start reaching sexual maturity, when the aggression really starts to show