As suggested above, it would be recommended to begin feeding whole organisms, such as fish, crayfish, and possibly chicks and rodents; size permitting. I am not a chelonian specialist so I cannot factually say what comes closest to a natural diet; but if you can simulate this your snapper would truely benefit. I have done some work experience at a zoo that fed it's adult common snapper rats.
It is a common belief that liver is a nutritional food source. While quite rich in many nutrients it is in fact a very poorly balanced portion of an animal; specifically when considering it's upside down Ca
ratio. The general rule of thumb when feeding carnivores is to assume that a healthy whole prey animal had everything it needed to live, thus it has everything the predator needs to live. Closely assimilating a natural diet (fish instead of chicken for example) may reduce tendencies towards obesity and lethargic behavior. A final note: I think that common snappers are somewhat omnivorous and alligator snappers wholey carnivorous or vice a versa; something to look into anyway 