Keeping your fillets cold is key. Some fish will keep longer than others. Bluegills for example get fishy very fast. Several days in a good cold refrigerator should be fine. Beyond that it needs to be frozen.

As far as freezing goes, oxygen is enemy #1. Oxygen causes a break down in the fats and oils in many types of meat. Fish is no exception. Oxygen causes these fats to turn rancid.

I have a vacuum sealer that allows me to fill my bags with enough water to cover the fillets and then I can seal them as well. This removes almost all the gasified oxygen. This prevents freezer burn and the degradation caused by oxygen. Keeping meats in a frost free freezer is also a way to quickly ruin meat, because these units heat up and slightly thaw the surface of items placed within them periodically in order to work. This will cause rapid freezer burn, which tastes as bad if not worse than the rancid condition.

My 2 cents on the subject.

Kent