First off, if your cat shouldn't have a diet at all of only tuna. The reason are the mercury levels in tuna. That is why pregnant women and small children are even advised to limit its consumption.

Second, I'm not sure how old your kitten/cat is. If it is a kitten, you could be doing more harm than good since it is not getting the daily nutrient requirements needed for its growth.

What you could do is start mixing a little tuna or tuna juice in with the regular cat food. Do this slowly and every other day add more cat food than tuna/juice.

Also be careful when you start introducing different foods to your cat. Cat's digestive systems take awhile to adjust and you have to introduce new food slowly.

Nakita on the other hand hates human food (including fish!) so she never begs for anything we are eating (except french fry's and potatoe chips!)