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What should the maximum water temperature for RES? Is 85 ok?

85 is warmer than we recommend except possibly for a small, sick turtle. For a healthy little turtle, 78 degrees is good. For an adult, 73 to 75 is good. When a turtle is sick we usually recommend somewhat warmer temps because it stimulates their immune system, but for healthy turtles, temps that are too warm will overstimulate their metabolism and cause excess skin shedding.

How big is your RES? A constant 85F is a bit too warm even for a sick RES. If your turtle is healthy and an not a hatchling, he should do fine with temps in the low 70s (my RES stays in water that's around 70F, except in the summer, which it's a struggle to keep the temp down). 78F for a small hatchling is good.

You didn't say where you are, but are you asking because it's warm where you are and the water temp is going up?

I would say that that is too warm. I keep squirts tank at a constant 78F.