Diet cat

I have 3 cats, a skinny cat, a heavy set cat and a fat cat. Fat cat has to go on a diet. So they’re all on a diet.



I truly believe that the diet kibble tastes as bad as human diet food. It even looks like cardboard. Fat cat isn’t really fond of it. Which I don’t mind.
Skinny cat really can’t lose any weight. It shouldn’t be a problem as long as I keep feeding her extra stuff. Problem is that she doesn’t really like extra stuff, just her dry kibble. She even seems to enjoy the diet stuff. How can I keep her from losing weight? I’ve tried the stuff for kittens (extra nutrients) but she keeps ignoring it.