sims96fan -
Items in your inventory can eventually go bad, but it literally takes generations for that to happen. I have a 12th generation legacy family and had some things finally go bad that had been handed down after a couple of deaths. For just normal play, I'd say it's safe to leave them in your inventory. They maintain their original level of quality, too.
For stocking ponds, I have found that the more you add to the pond, the better the fishing will be. This makes me think that the fish do not breed. I've been able to get a pond pretty well stocked, but haven't fished much out of it to know for sure.
As for the quality of the fish coming from the ponds, I believe it's related to the level of your fisherman. Upper levels catch better quality fish, no matter where they are.