You're right, this is an important question. One that is close to my heart too.

Algal oil may be the best bet. I've decided that when my krill oil runs out I'll be switching to algal oil. Here's something from Purdue pharma (yes them) they did a little trial on a few different versions. Algal oil was best by far. Easy read infographic. https://www.purdue.edu/hhs/nutr/fish4health/PurdueResearch/CapsulPCB03.pdf

Krill oil is a better option than generic fish oil or cod liver oil. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4179167/

Here's an interesting article that's relevant for those in the US which seems positive. https://www.ebmconsult.com/articles/fish-oil-omega-fatty-acids-supplements-mercury-toxins

You're right to be cautious about the toxins in fish. It's sad that we've done that to our oceans. Disgusting really.