Too much fish, or meat, will cause failure in the kidneys/liver. I can't recall which one it is. I know, as it happened to my kitty, Tex. He prefers meat to all else, and this last time, his BUN level was high. The vet said it is a sign of too much protein in his diet.

So, I talked to the vet about this. He said it is NOT a balanced diet for kitties, and try to find a commercial food that will cover Tex's nutritional needs. (He also has other health issues, so, he is on a RX diet).

I would strongly discourage you from feeding your kitten a mainly fish diet (or meat, or veggie(unless you are balanced), etc). It isn't balanced, and isn't good for the kitty.