Tank Size - The Bigger the Better
Water quality and cleanliness is easier to maintain in a larger tank. In a smaller amount of water the waste products more concentrated. With a larger tank, waste matter and its by-products are diluted. In a larger tank, partial water changes are more practical for maintaining consistent water quality, rather than having to change a large proportion (or all) of the water in a smaller tank. A general guideline often quoted is 10 gallons per inch of turtle.
There are several options for filters. When it comes to turtles, choose a filter rated for 2-3 times the size of your turtle tank (i.e. if you have a 20 gallon tank, choose a filter rated for 60 gallons, even if the tank is not full). Filters with several different levels for removing waste matter as well as by-products are recommended (i.e. mechanical, biological, and chemical filtration). The topic of filters can seem complicated and daunting, but the pages listed below cover the pros and cons of different filtration methods as well as tips on maximizing the benefits of filters:
-Filter Types - RedEarSlider.com
-Filtration - Austin's Turtle Page
Partial Water Changes
Regularly take out part of the water and replace it with fresh water. This removes and dilutes waste products. The frequency of partial changes and how much water you need to change out will vary, depending on factors including the size of your turtle(s), the size of the tank, the filter, and whether you feed in the tank. Frequent partial water changes (weekly, perhaps 2-3 times a week if necessary) will do a lot to help keep the water quality high. Using a gravel vacuum or a siphon to remove water makes this job a lot easier, but never prime a siphon by mouth due to the risk of Salmonella contamination.
Skip the Substrate
Keeping the bottom of the tank bare makes cleaning easier since wastes and uneated food can't get trapped in the rocks. Rocks or large gravel (too big to be ingested) at the bottom of a tank can be attractive, but aren't necessary.
Feed Out of the Tank
One way to reduce the amount of waste you need to manage in the tank is to feed your turtle in a separate container, though this is a matter of choice. A smaller plastic tub or storage container can be used. Using water from the tank is an easy way to make sure the temperature of the water is warm enough; just replace the water taken out for feeding with fresh water (and you've done a partial water change at each feeding). This eliminates the problem of excess food decaying in the tank, and turtles often go to the bathroom shortly after eating so the amount of turtle waste accumulating in the tank is reduced as well. Then you can just clean and sanitize the lightweight feeding container after each feeding. However, this is a lot of additional work and the extra handling may be stressful. You may choose to the separate feeding tub for messier or higher protein meals, and feed other less messy foods like greens and vegetables in the tank. Many owners decide to feed in the tank too, which is fine especially with a good system of filtration, water changes and monitoring. Scooping out excess food particles and doing water changes shortly after feeding can also help if you feed in the tank.
References and Resources