Fish oil should be nearly 100% pure. Purity will depend on the manufacturing process and there is a lot about that you need to know about. The best fish oil starts with premium sardines, anchovies, tuna or salmon. Other kinds of fish will produce a lesser quality fish oil. The fish parts used needs to be flesh, excluding the tail and the head. Cheaper fish oils have been sourced from fish leftovers so be wary.
Aside from the kind of fish, the resulting fish oil has to have been screened and shown to be with out disease. After all, fish can get sick too. The manufacturer should be able to show where their fish is harvested from. Some seas are considered too polluted, yielding contaminated fish.
The fish oil you buy should not have been passed through a molecular distilling process. Fish oil is sometimes distilled this way to remove toxins from contaminated fish. But the process will also destroy the oils natural form.