I wonder if that's because the fish are so old -- maybe the game can't deal with 5 digit ages? (so max. age = 9999 years?).
Since your fish are so old, I guess you are using the "set your system clock ahead" cheat to make the game progress faster -- and since the game wasn't intended to work that way, the fish get much older than expected. Or the game gets confused if you set the clock back to the real time.
I think the only way to avoid this is not to mess with the system clock -- or sell the fish before they get to 9999 years
.
-- jen (stickin' with real time, which I don't have enough of anyway!)