I agree with modernhamlet 0 we shouldn't flush (not only because it can harm but because to me it seems dis-respectful) - I know its only a fish but if you didnt care for it then you wouldnt spend ages feeding it etc.
Just put what you usually do!
Thankfully I haven't had any deaths yet - but I may bury them assuming my cats won't becoming interested!
Whenever this sad event happens, I either put the poor thing in the nearest wastepaper basket or flush it. It just depends on which is closer to the tank.
I bury mine, first powdering them with something very bitter or very hot (spicy) since these two flavors do not usually appeal to skunks, raccoons, and feral cats. Most fish that I've had died were treated with antibiotics prior to dying. Some of those antibiotics contain substances that could harm animals who injest them, so making them smell unappealing will decrease the chance of them being dug up and eaten. Deep burial depth and a rock on top also help.
My uncle had a bullhead catfish he caught from a lake as a fry. He had if for a very long time. I forget how long, but I believe it was over 20 years. He had it creamated O_O
I freeze my dead fish, seal it in a plastic bag and then put it in an airtight container, I have tons lying around and then throw away the fish in the trash.
But my family buried my two goldfish after Katrina in our yard, and we will plant a rose bush there when I come. They were too special. Not that my other fish aren't special, but they were uber special.
I never thought of them harming the environment by flushing them- good job i dont like it! I prefer wrapping them up and putting them in the bin. I know a mate that flushes his, but hes recently got some discus... so i wonder what will happen if they die.... Not sure the pipes would enjoy that!
I still done have any fish, i'm currently cycling, but whenever i have any deaths of my fishys, i think i will bury them since its sand, so i can easily make a pretty deep hole