Many people will say that some fish, goldfish in particular, will “grow to the size of their container.” If given lots of room and food, carp species, including goldfish and koi, have the potential for ridiculous growth, up to 16-20″! Many of the largest goldfish even seen were once pets that were “released” into the wild. (NOTE: NEVER release your pets into the wild. If they manage to survive, they often decimate wild populations. )
So, why does your 10 year old goldfish in a 10 gallon tank never get any bigger?