Apart from the issues commented by Alioth, tuna is a salt water species whereas axolotls are exclusively fresh water, so the salt contents and various elements are quite different. Also, vertebrate muscle is not easily and fully digested by most caudates, it´s way too caloric and nutritionally incomplete. In other words, it´s a completely unnatural food that provides very few benefits and could produce a variety of problems. Stick to earthworms and other invertebrates, that´s their correct diet.