Hi all. New to the forums here.
My son has a RES about 2 years old. It's maybe 6 inches long. It's in a 40 gallon tank, I think. Temperature is around 72 I believe with a heater and a in tank flyer with a coarse sponge (not sure how helpful it is)
We weekly do a half tank water replacement and use turtle clean with the changes.
We use a UV light, have a styrofoam basking platform with a heating coil heater.
He's been healthy and still seems well but this week, when I've been left to look after him (my son is at camp) the tank water has gotten very dirty and has an unpleasant odor. The turtle is eating less than when my son feeds him but is still eating, most recently this morning.
I've changed the water twice in the past 2 days. I suspect I may have over fed him and the food may be contributing but I've fed him and haven't gone overboard. I also thought a nearby window usually has the blinds closed but was open for ventilation may have caused algae overgrowth so I closed it.
Any other thoughts? Thanks in advance!

