Dechlorination of the water is not necessary, chlorinated water may help keep bacterial levels down. Young turtles and those not accustomed to chlorinated water squint their eyes a lot at first but quickly get over this. Water should be at least as deep as the width of the widest turtle's shell so that if overturned, the turtle will be able to right itself and avoid drowning. Diamond back terrapins live in brackish water and require addition of 2 tablespoons of aquarium salt and mineral mix per gallon of water. Filtration can decrease the frequency of complete water changes but not eliminate them.
Turtles produce considerably more solid fecal waste than fish.
Most retail aquarium supply stores sell filters designed for biologic breakdown of fish waste not turtle waste. Keep in mind that even if the water looks clean, it can still have a lot of nitrogeneous waste in it. While many species can exist at lower temperatures, they prefer and will do better at warmer temperatures. Therefore, submersible aquarium heaters are needed. Use a liquid crystal display thermometer outside the tank to monitor temperature.