I have a 6 inch RES named Bitey and he lives in a 135 gallon tank. I feed him once a day, and alternate between pellets and vegetables (romain lettuce or spinach usually). He also gets turtle treats maybe once a week.
Just recently he has started attacking the silicone lining of the tank - he'll get as big a bite into it as he can and then pull back. He eventually lets go and goes flying back a little, and the silicone makes a loud snapping noise. I'm partly worried that he'll eventually tear up the lining and make the tank leak, and I'm unsure if this is normal RES behavior or indicative of something missing in his diet.
He isn't displaying any other odd behavior (has a good appetite, basks regularly, swims around the tank, etc.).






