Thats rubbish! Where do you think your poop goes - straight into the local waterways? Everything that is flushed goes to the sewage plant and is made safe - and thats where your flushed fish will go too!
Think about it
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Of course she knows poop goes down the loo too
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But tropical fish are shipped from all over the world, and often carry different diseases/parasites from those found here. Just because the fish is dead, doesn't mean the disease that killed it is. Even a fish that didn't die from a disease could easily still carry something.
Just look at the crayfish plague, or that virus grey squirrels carry. It doesn't effect the non-native animals because it has a resistance to it, but our native ones don't.