No 'cheap fish oil', or 'pharmaceutical fish oil'
Cheap fish oil is not fractionated and has not been subjected to chemical molecular distillation, but it contains far too many PCBs, dioxins, heavy metals and methylmercury.
Do the test: put a dish of fish oil in the fridge for five minutes (split open a capsule). If the oil remains liquid, you have 'pharmaceutical quality', that you should avoid like the plague because it has been processed far too much. If the fish oil solidifies, then you have unprocessed fish oil that you should also avoid nowadays because of pollution.