I realize that I probably should have thought of this sooner, but I put Mort in this tank during the summer, when the room he's in is usually 80F, and didn't consider adding a heat source. I set a red heat bulb on top of his tank yesterday, and he's been basking ever since. I'm hoping that this helps him digest. I'll feed him again tonight a and post how it goes. Just going to try minnows tonight, and I'll throw in a mouse or two next time.