My cat is finicky. He has only a mid priced prefered wet food:
Whiskers, which smells disgusting to me and does not seem very healthy.
He does not eat canned Sardine either, but loves Tuna fishes.
So to give him a better food quality for less price, I often oven frozen Alaska pollock and then smash it to pieces by fork to make cat food. But the cat hardly eats that supposedly healthy food after the first day or two of cooking.
Sometimes I add tomato paste or dried basil or a bit of salt, hoping to make the fish more appealing. But they don't seem to help much.
It is worth mentioning that the cat has had upper respiratory disease, and has frequent dark discharges from his nose. So maybe his sense of smell is not great.
Nevertheless, I'm wondering if they are some ingredients which are particularly liked by cats to add to his fish food?