If you have an adult fish that is very sick, and it will die soon, you can still sell it. Many customers will have a thought bubble showing that they won't buy it, but eventually, someone will.
What I do is set my play time to stop. You can still sell fish that way, and it won't die while you wait for a buyer. I forgot to feed a bunch of goldbulbs and still sold them all before they croaked.
A note to the programmers -- does time stop in the fish shop anyhow?

I've never had one die while in the shop, but I didn't know if that was just a coincidence.
cheezfri