Why does water quality matter? Because water turtles spend most of their life in the water, and they also drink the water in which they live. The simple answer to the title question is: The water gets dirty. But aren't bogs and rivers dirty, too? The water there is often dark, and it often smells. This is where one must discern between healthy waters and polluted waters. A healthy bog/swamp/pond is a balanced ecosystem. The water, plants, dirt, algae, and animals that live there are in balance. The murky smell from decaying plants or just "dark" water may seem strong to our noses, but many animals, including some turtle species, live in such waters. The larger the body of water, the more water gets exchanged (in nature, by rain and creeks), the more filtration there is (in nature by the earth bottom and the plants), the better the water quality. If the body of water gets out of balance, for example, because of fish or turtle overpopulation, then the ecosystem collapses, and those animals that cannot emigrate to a new home, will die. That simple. Today, a lot of the waters in which turtles life are polluted. Pollution can be industrial and/or residential (sewage). Studies show, that turtles in polluted waters have a higher incidence of skin diseases, shell diseases, other diseases, and cancer. Often the eggs won't hatch, or the babies are deformed. In the wild, sick turtles usually die, often slowly and painfully. So, why do turtles live in bad water places? They don't know better. An aquarium is a closed system of water, plants, animals, and the filtration system. It is true that the bigger a tank/aquarium is, and the more filtration there is, the less often one must change the water. However, any tank is a closed system, and the water will eventually spoil. I have a 400 liter fishtank with fish and plants--a wonderful little ecosystem, with good filtration and all. Even there, I have to do a partial water change about once a month. Why? Several things happen in all standing, inhabited water: * Ammonia increases due to waste products. Ammonia, in higher concentrations, is bad for the skin and shell of the turtles. (Think AJAX cleaners!) At the latest when you can smell the ammonia (that smell is different from a healthy bog smell which you can get in a well-balanced brackish water aquarium) it is high time to change the water. * Bacteria grow. Those bacteria will attack the turtle's shell and skin, leading to skin rashes and shell rot. When the turtle drinks the water, they get into the respiratory and digestive systems and can make the turtle sick. * More things happen to the water chemistry, including changes in pH, Nitrate and Nitrite concentrations, smell, algae growth, etc. They are secondary to the two above, but still matter. Why does the water get dirty? * Food. Food leftovers decay and soil the water. You can minimize pollution from food by feeding the turtles in a separate enclosure and by removing uneaten food from the tank after the turtles stop eating. * Waste. Turtles pee and poop into the water. The amount of pee and poop is directly proportional to the number of turtles and the amounts they are fed. Waste and waste decomposition generate Nitrates, Nitrites, and Ammonia. How can you keep the water free from Ammonia and other pollutants? * If you had a huge filtration system that removed all ammonia, regularly checked and adjusted the pH, always removed solid wastes, then you could go for a long time without changing the water. For example, if you put a canister filter designed for a 100 gallon aquarium on a 20 gallon turtle tank, used filter media to remove Ammonia and particles, cultivated a filter environment that is balanced, tested the water every week to make sure everything is OK, then you could go for much longer without changing the water. However, you would have to clean the filter periodically and replace used up media every few weeks. (Note that cleaning a canister filter takes longer than changing the water; and filter media are more expensive than a hose.) * If you used a water reclamation system, like they do on the space shuttle, then you never have to change the water. Such a system costs several millions of dollars. * The easiest and cheapest and simplest way to keep the water clean is to change it regularly. That way you guarantee that the water is of good quality. You can use a filter to extend the amount of time between changes. How often you have to change the water depends on the size of the tank and the number of turtles. (I change the water when it "looks dirty.") If you need more convincing, that changing the water before it stinks is a good idea, please, check out some comprehensive aquarium books. Any serious aquarium book has a chapter on water quality. Or, imagine taking a bath in your toilet, after using it. Not a healthy thing to do. But that is what a turtle in a tank is basically doing. What about low pH? ------------------ Someone mentioned, that turtles like low pH. I am not sure about that. What I do know is that a lower pH discourages some bacterial growth. The way to get a lower pH is NOT to avoid changing the water. Instead, you can use aquarium chemicals that will adjust the pH to a lower setting and keep it there. Please, refer to an aquarium book, rec.aquaria, or an aquarium specialty store for more information. Here is what F.L. Frye has to say about pH in "Reptile Care, an Atlas of Diseases and Treatments": "Some authorities have recommended acidifying the water supplies for reptiles. The theory behind this recommendation is that the highly pathogenic organisms, Pseudomanus sp. and Aeromanas sp., that are so commonly isolated from sick reptiles and their environments prefer a more alkaline medium in which to grow. By lowering the hydrogen ion concentration, or pH, of the water to 5.0-5.5, the growth of these microorganisms can be suppressed. Verious organic acids can be used to decrease the pH, but phosphoric acid seems to be the most readily accepted because it does not impare an objectionable taste to the treated water."