I agree with Trillian

I think being open about it is the best policy. Saying that something/someone has died and why they have died and saying that it means that they won't be here anymore is actually really easy for a child to accept.

Experts generally say that kids have no fathoming of the finality of death until they are about 8 or 9 at the very earliest.

I think that going on a little expedition to pick a new fish would solve the immediate problem if you feel that he is too young for the full discussion.