My adult female RES (maybe about 7 years old) has been behaving a little differently lately, and I wonder if there is something wrong with her. She shares a 100 gallon tank with another adult female.

1. She spends more time during the day burrowed in the gravel substrate at the bottom of the tank.
2. She is basking less than usual.
3. BUT, she looks fine, and . . .
4. She still has her voracious appetite.

Any ideas? Thanks in advance!