I think that most people who feed fish fillets occasionally supplement it with calcium and vitamin/mineral powder. Personally, I'm a big advocate of occasionally supplementing any diet with vitamins and minerals and providing a bit of variety once in a while. I also think that feeding fillet strips from an adult salmon would be much more likely to be high in fats/oils than parr since the parr are growing rapidly and haven't accumulated rich fat stores, yet. In fact, the omega-3 fatty acid that salmon are so famous far is only accumulated at sea when salmon eat phytoplankton and/or the consumers of phytoplankton higher up the food web. Most of the fat can be trimmed off the mid-section of a fillet very easily.