I've noticed that my RES has been resting on the bottom of tank in the same general area the last several weeks. She still is eating, though not as readily prior to the change. In addition, she still defecating. She still basks and swims about, but also not as often.
I do weigh and measure her once a month and she has gained weight since the last time.
I do ~50% water changes about every 2 weeks and check the chemistry after each one. My water is hard, but otherwise nitrate, nitrite, and ammonia are low or none.
I suspect it may be due to the fact it's winter here in SW Michigan. It gets gloomy here for people. The tank is in a room with skylights and is on the north side of the house, so currently, not much sunlight comes in. I reduced the duration of the lamps a few months ago as well to about 11 hours, though I added maybe 30 minutes today.
Again, I do suspect it's due to it being winter and perhaps not noticing changes past year as I've had the opportunity to work from home more often and can watch her throughout the day.
Anything else likely possible?