Some people think goldfish are safe to release (the influx of them into a local lake would suggest this). If any of those people are here, the max size of a goldfish is more than 12 inches. In a couple generations they lose their nice colors and turn brown. Goldfish are closely related to carp. Do you see where this is going? Even worse are koi. The same process goes on with them, except koi are even more similar to carp, growing to a size of 36". The amount of carp present in the US and Canada is too high. Common carp and Asian carp are not native. Neither are koi and goldfish.
To give a clue to the problem near me, and this isn't because of aquarists I will admit, is huge. The worst lake has an estimated 20,000 (might be 200,000 but that sounds like a ridiculous population so I will go with 20k) common carp, the odd goldfish or koi, largemouth bass, smallmouth bass, pumpkinseed sunfish, brown bullhead, rainbow trout, cutthroat trout, steelhead, yellow perch, and maybe more. The native species? Cutthroat trout, rainbow trout, steelhead. With most of the common carp under 10lbs, I can't help but wonder if they are not all common, and some are escaped koi or goldfish. Some of those introduced species are in pretty much every water system in the area but largemouth bass, and the carp are exclusive to this lake. The native species are rare, particularly cutthroat trout which are not stocked. The only gov't stocked fish to my knowledge is rainbow trout, for the fishermen. So there are 6-8 introduced species there. That is just wrong. And proof that introducing fish wreaks havoc upon an ecosystem.