Like Sharky said, tuna is fine but only as a once-in-a-while treat.
My cats poop every 1-2 days, but usually every day. Is your cat drinking a lot of water? Usually when a cat eats primarily dry food they aren't getting enough water. I would look at water fountain's like the PetMate Fresh Flow ones. My cats drink a LOT more water with these then they do when I just put it out in a dish.
I would try to get your cat to eat more wet food, what wet food brands have you tried feeding it?
My cats poop twice a day on average, and they eat mostly dry food (EVO) and some wet food every morning. I think most cats poop everyday, like humans. Firm is different than hard, maybe some hairball meds are in order?
Have you tried canned chicken? My cats eat that sometimes as a treat. Just some ideas
She eats canned tuna (1 spoon a day) to get her pill, canned 9 Lives or Sheba and dry Indoor Berkley Jensen or Deli Cat. We cannot get many choices of food on island but I am going to the US Sept. for 3 weeks so can get what you recommend. She is an indoor cat. She pees 2x a day and I never see her drink but am not home all the time. THANKS
My baby-cat poops once in 2 days ever since I switched from wet & dry to dry only. Seems she does drink more water with dry-only but now she stopped pooping every day.
When she was fed with wet & dry, her poo poo got kinda loose & stinky.
My vet told me to cut down on wet to prevent that. I dunno which is better for her, though...