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sesame, who's about 5 years old, has always been super active, always swimming around, trying to get out, and always takes pellets from my hand. but over the past week or two he's been acting unusual - not taking pellets from me anymore, starting to eat less (but not stopping altogether), won't touch lettuce, chewed up and spit out a strawberry even. when he's out and about he's not quite as active, but still looks alert and healthy. he's been sleeping quite a bit during the daytime underneath a log in his tank, and at nighttime has taken to sleeping up on his dock, neither of which he'd done very much in the past. at times he also seems easily startled. i just changed his filter a couple weeks ago and changed out a bit more than half the water last weekend, and except for some algae his tank is overall pretty clean.

i wondered if this might be a seasonal type thing developing (he's nearly 5 years old now), or might he be sick? since he's still eating and moving around enough i'm not terribly worried and jumping to a vet, but if there's reason to go to one i hope someone can let me know!

Are all your temps normal? Water and basking area?

yup, no changes to either, water consistently at 77-78

It can be a seasonal thing... you might want to see his reaction to treats. And what's his reaction to people... still the same? As a precaution, you might want to bump up the water and basking temps a little.

What kinda thermometer do you use? I use one with a wire and probe that you stick somewhere in the water with a suction cup. I hear the stick on the outside of tank ones are a little inaccurate. I would listen to Steve and up the temp a few degrees.