There's a difference between fishy and rancid. Rancid will be a putrid, slightly sweet odor. It will not be an appetizing scent. If you think it smells rancid (whether a lot or a little), then don't eat it.
If it's been completely frozen, then, if it's rancid, it will have to have become rancid before freezing. It may become freezer burnt if not properly wrapped, but that's a different matter altogether, and has to do with flavor and consistency rather than safety (at least to the best of my knowledge).
Fishy is, well, fishy. Fishy is certainly not good - it means the fish isn't terribly fresh, and won't be as tasty as fresher fish - but it's not necessarily dangerous.