I make 40 fishcakes at a time (freeze them cooked, and oven-bake from frozen) and use part Pollock/coley/similar, part smoked haddock, and then a tin of salmon/sardines/mackerel for the fish oil.
I tend to buy odd bits of fish here and there, eg if reduced, and freeze the individual fillets, then poach them in milk before mixing with the mash. The fishy milk is then used to make fishy white sauce to freeze (for future fish pie, or salmon-en-croute)
50:50 fish:mash, coated in flour, egg, breadcrumbs/panko and fried. Kids love them. Take about 25 mins to oven-cook from frozen (same as frozen chips, coincidentally!)