You do need to be careful what fish oil products you use. There is always some contaminated product floating around out there.

It needs to be molecularly distilled to be free of heavy metals and dioxins. Also needs to be fresh. I'd actually recommend puncturing one or more of the capsules and smelling the oil. If it smells really fishy, it's probably rancid. That would mean that the manufacturer was not careful enough, didn't process the oil quick enough, etc. Fresh oil should not have any strong fishy smell any more than fresh fish would.

Capsules should be kep't in the fridge after opening to be on the safe side and keep them their freshest.

Keep these guidelines in mind with all your oils, including the cooking oils. Oils high in polyunsaturated, expecially, should be kept refrigerated. If they are higher in monounsaturated, like olive oil, they can be kept room temperature, to a point.

With that said, fish oil is a great thing to be taking.