Try different flavors of the FF, it may affect the smell of their poop. Try each flavor for a couple of days or more, so you can tell what the effect of that flavor is. I have one cat who is sensitive/allergic to chicken. She gets food with chicken in it, her poop stinks to high heavens and she gets diarrhea. We feed her foods with no chicken, and there's no diarrhea and very little smell. Another cat we have cannot eat fish, he gets diarrhea and the smell will drive us out of the house. No fish in his food, things clear up and his poop hardly smells. Our third cat can eat anything, nothing bothers him.
(We saw the same thing with our dog. When we adopted him, he was, of course, on the cheap dog food that shelters buy so they can afford to feed so many animals. He was a fart machine, and the smell would actually drive us from the room. Deadly. Beyond Deadly. Unreal. We got him switched over to a better food, a limited ingredient one (he also had itching issues) and the farting stopped. Completely. He might fart once a year now, if that, and that's usually following a treat he really shouldn't have had. The food they eat makes a HUGE difference in the smell of an animal's poop.)
