How much feeding of fish varieties of cat food is ok?

I know this is quite a debate. I have a 15 yr old male cat that has been underweight. He looks like he is putting on some weight but I have been feeding him more FF fish 'flavored' foods. He gobbles those up.

What should I do at this point? I don't want to have him on an all fish diet. I also want to give him other varieties so he won't refuse to eat anything but fish.

How much fish is safe for a cat?

Once or twice a week is enough and won't cause any harm, IMO.  Can you mix a little seafood caned food with some meat canned food as a sort of topper? Or use crushed up seafood freeze dried teats?