Flushing live fish is terrible, just as bad as those who flush kittens and other things down the can imo.
On a side note, don't release your freshwater fish into streams if you can't get rid of them either! Many of them DO survive.
I know a place where there are schools of goldfish swimming in a stream here in Ontario. It's pretty crazy, but yeah.. introduced. I can imagine they have ruined the lives of some other little pond creature.