Too muc htuna is bad. Try mixing a little in her regular food and then day by day reducing hte amount you mix in. I am pretty sure that tuna should only be a treat, but I could be wrong.
regular tuna can deplete viatimins in kitty ... best thing to do is give kitty tuna flavored cat food as the viatmin issues are taken care of via added vitamins... what brands are you feeding if I can ask??
If you don't offer her tuna she won't have a choice and will get hungry and eat what you offer her. I know it sounds harsh but as long as you offer her the tuna she is going to eat that over everything else. Try tuna flavored pouches or cans.
My 17 yr old gave paws up to Solid golds tuna cat food .... its main ingrediant is tuna and then vitamins ... small mom and pop shops some vets and some petcos have it ...