I think almost every fish species can become overabundant or overpopulated and stunted if there are no or not enough check and balances present. Just about every fish you can think of can at times become a problem. Some fish definately do more habitat harm than others. This seems esp true with most invasive/exotic species but not all.
Tyw33: Are you telling me that if I live in MN and if I have a private backyard pond (say 1/2ac w/ no access to a stream) and if I put gold fish in it, that the state fishery boys or the local game warden will come and make me take them out? No gold fish, not even in private ponds?
Goldfish become overabundant in the city, metro, or urban lakes because all the yocals, excuse me locals, take out the bass who if present in normal densities would control and probably eliminate the goldfish.
Goldfish populations do fairly quickly become drab colored for two basic reasons. 1. A sizable percentage of goldfish offspring is drab or mixed color because drab is the dominant gene. 2. Colored offspring stick out like sore thumbs or neon lights and predators get them first.
