Once precaution must be taken before each water change. The heater MUST be turned off and removed form tank beforehand. Else you risk cracking it if there's a change in water temperature. Keep in mind that the air temperature must be warm also. Heat the room were the turtles are kept in the winter, and do not air condition it in the summer.
Hoods also help to keep the air warm.
A dry basking area is essential to the well being of the red-eared sliders. Basking is one of their main activities and they must get dry while benefiting from the light you provide from time to time to prevent many ill states like fungal infections, shell rot and eye infections. Lightning Red eared sliders need full spectrum light that emits UVB to process nutrients of food you provide. This absorption encourages proper bone formation and shell health. Simulating a day and night cycle with it will also help your turtle stay healthy.