The water The water used to fill your turtle aquarium should of course be clean, but in most areas tap water is clean enough. In the wild freshwater turtles live in rather hard and alkaline waters, and rainwater is usually too soft and acidic to be suitable. The pH should be kept above 7, but this is usually not a problem since tap water is slightly alkaline. If you want to be extra safe you can put a turtle neutraliser block in the aquarium. If you dont perform regular water changes the pH can drop to acidic levels, which will cause injuries to the shell and skin of your turtle and sometimes lead to fungal infections.
Turtles are sensitive to heavy metals and its therefore a good idea to use a water conditioner capable of removing such impurities. Most water conditioners will also remove chlorine, and even though freshwater turtles are not as sensitive to chlorine as fish are, its still a good thing to get rid of it.
To keep the aquarium nice and clean for you turtle, a filtered aquarium will require partial water changes of approximately 25% of the aquarium volume every 2 to 3 weeks. Clean the filter media by rinsing it and use a siphon to remove any debris from the aquarium. If you dont use a filter its advisable to siphon out debris every few days and do a 25% water exchange once a week.