illegal to flush a fish are you kidding??
If you keep none native fish and plants in an aquarium or pond under
no circumstances are either to be disposed of in a situation where they
may affect the native fish or plants, especially if they died through a
disease. You must take all the steps necessary to ensure that both non-
native fish and plants are disposed of in the correct manner
So, no flushing, throwing in a river, dumping in a pond etc