The risk with fish oil is the pollution and heavy metals they can contain, can you actually now the origin of those oils ? What fish, where has it been caught etc and/or its safety ?
In my opinion, I think it's more likely to just be money down the drain than it is to actually be significantly harmful.
But if you're going to do it I would suggest buying it somewhere like Costco. If you've ever been to Costco you know they usually only have one brand of everything and most of what they carry has been carefully chosen to be the one they carry. I'm sure they've made mistakes but I have been very satisfied with the multivitamins I have bought there.
Obviously, it's entirely subjective as to whether anything does anything or not. I've tried fish oil and didn't notice a difference in anything so I stopped throwing money away. I started taking multivitamins and noticed a significant difference in my overall health. Was not a miracle for SA but I'm skeptical about everything. I believe multivitamins work if you get a decent one.