Once your turtle tank is fully functional, the water in it will be kept clean by "friendly" bacteria who eat the ammonia and other harmful chemicals in the water and convert them to less toxic chemicals. We call the process of getting those bacteria established "cycling," and the process itself is called the "nitrification cycle."
When you first set up a new tank, you need to do a few things to make it suitable for your turtles. One of those things is removing the chlorine and chloramine if your tap water contains those chemicals. There are additives called aquarium dechlorinators that you can buy from Amazon or a pet store that will help do this. Just letting the tank sit for a few days with the aerator on will also effectively remove chlorine, but not chloramine; so if your tap water contains chloramine (or if you don't know if it does), you should use a dechlorinator.
The next thing that needs to happen is to get the friendly bacteria up and working. The friendly bacteria will live mainly in the biological media of the filter, and some will live in the substrate and other objects in the tank, if you have any.
Please note that you'll need an aquarium test kit to perform the water quality tests on this page. You need to have one before you introduce the turtles to the tank to make sure the water is safe, and to test the water regularly thereafter.
I've received some criticism regarding pervious versions of this page for not adequately explaining the roles of the friendly bacteria in an aquatic habitat, so with this revision I'm going into a bit more detail.
Getting the bacteria up and working is a vital part of setting up a new turtle habitat. But there's a catch to this part of the job: The turtles need the friendly bacteria to keep their water clean, but the friendly bacteria need the waste the turtles produce in order to live. That means that to get the friendly bacteria, you either need to put the turtles in the tank before it's cycled and stabilized (bad idea), or get the bacteria working some other way before putting the turtles in the tank (good idea).
To better understand the things that have to happen to get your turtle tank "cycled," you need to understand how the nitrification cycle works. The way the nitrification cycle works is like this:
The simplest (and slowest) way to get the process started is to simply set up the tank, fill it with water, start the filter, and add small amount of fish or turtle food to the water. You can also use household ammonia instead of fish or turtle food if you like, starting with one drop for every two gallons of water. If you use ammonia, use plain household ammonia, not some detergent or cleaning process that contains ammonia. You want a product that contains nothing except ammonia and water.
After you add the food or ammonia, wait a couple of days and check the water chemistry. If there's no ammonia showing, ad a little more food or ammonia and wait again. Eventually you should see an ammonia spike, which means the process has started. At that point, you'll want to check the water chemistry every day and add a bit more food or ammonia as needed to keep the ammonia level between 3 and 5 ppm (parts per million).
At some point, you'll see a nitrite spike, which is a good sign. It means that the Nitrosomonas bacteria are starting to do their jobs. Once you see that spike, keep feeding them a little fish or turtle food every day and checking the water chemistry, again aiming for an ammonia level between 3 and 5 ppm.
When you see the nitrite levels go down and the nitrate levels go up, that means that the Nitrobacter bacteria have shown up for work, and your tank is nearly or completely cycled.
The problem with this method is that it usually takes a few days for this process to start in a new turtle tank, and a few weeks to a few months before enough of both groups of bacteria have colonized the tank and filter to complete the cycle and make it stable. During this time, the ammonia, nitrite, and nitrate levels may skyrocket to levels that can harm your turtles and any other creatures that you keep in the tank. That's why it's important to check your water chemistry several times a day if you decide to put your turtles or other living things (except plants) in the tank before the cycling is well-established.
If your ammonia, nitrite, or nitrate levels go high during this time, and you do have turtle or other animals in the tank, then siphon off one-quarter to one-half of the water in the tank and replace it with fresh water to reduce the amount of toxic chemicals in the tank. Do that several times a day, if necessary, until the ammonia, nitrites, and nitrates are down to safe levels.
Ultimately, if we only have turtles in the tank, what we're shooting for are these numbers:
If you have fish or other aquatic creatures in the tank in addition to your turtles, then you'll need to follow the guidelines for those animals. Fish, amphibians, and crustaceans tend to be much more sensitive to water-chemistry issues than turtles are.
During initial cycling, the water-chemistry may be all over the place. That's normal and part of the process. If you check your levels frequently, this is the order in which you should see things happening:
It's very common for the water in a turtle tank to go cloudy during the initial cycling process. Don't worry about that. It's what we call a bacteria "bloom," and it's normal during cycling. The bacteria need the turtles to produce the ammonia and wastes that the bacteria eat, and the turtles need the bacteria to help keep the tank clean; and it takes a while for all this to balance out.
UV Sterilizers or clarifiers use Ultraviolet C light to destroy algae, most bacteria, and many viruses, fungi, and parasites that are exposed to the light. Usually they don't bother friendly bacteria because most of them live inside the filter and on the substrate, not freely in the water.
Once your turtle tank has cycled and stabilized, UV sterilizers are good things to have. They help to keep your water clean and free of algae, and they help kill bad bacteria and other things that can hurt your turtles. Many filters have UV sterilizers built right into the filter for that reason. During cycling, however, the good bacteria haven't finished moving in to the filter, so it's very important that you not use a UV sterilizer during cycling. If a UV sterilizer is built into your filter, leave it turned off during cycling. This is also important if you replace your filter or change the media inside of it. If you kill the friendly bacteria before they can get to the filter media, they will never be able to establish a colony there.
If you like, you may be able to speed up the cycling process a bit. If you already have your turtle(s) and need to get them into the habitat, then you may have no choice other than to use one of the following quick-start cycling methods.
The first way is to introduce something into the turtle tank from another turtle tank that is healthy and fully-cycled. There are four ways you can do this.
If you choose to introduce friendly bacteria into your tank using any of these methods, make sure that the habitat they're coming from is healthy. Also make sure to make the change quickly once you do it. If possible, no more than 15 or 20 minutes should elapse without the bacteria receiving aeration. If you're moving filter media or gravel from the existing thank to the new one, transport it covered in water from the tank you took it out of. If it will take more than 20 minutes to get it into the new filter or tank, aerate it with a portable air pump to keep the bacteria alive during the trip.
If you don't have access to another established habitat, try asking local pet stores that sell fish if they can give (or sell) you some cycled filter media. Some will, and some won't.
The second way to speed up cycling is to use a bacteria supplement made for turtle tanks, fish tanks, or fish ponds as "starter bacteria." I usually use a bacteria supplement made for fish ponds, but only because I have a lot of aquatic habitats and the pond bacteria is much less expensive than the supplements made for tank habitats. The bacteria are basically the same, or at least close enough that I've never had any problems using it in tank habitats.
If you're only setting up one tank, however, then I suggest that you use Tetra SafeStart instead. It's specifically made for freshwater tank habitats and is easier to measure and use. Proper dosing is also much easier with SafeStart because it's not as concentrated as the pond supplements are.
If you choose to use a bacteria supplement, you should add it at about the same time that you first add the turtles. Remember that the bacteria need the wastes produced by the turtles, so it would be a waste of money to put it in a tank with no occupants. The bacteria will die without the turtles (or fish, if the tank also has fish) in the tank.
By using starter bacteria, you probably won't see such high spikes of ammonia or nitrite because there already will be bacteria waiting to eat it as soon as the turtles start producing it. But because those high spikes are part of the normal cycling process, using starter bacteria may actually cause it to take longer for your tank to become completely stable on its own. It just won't have such extreme spikes while it's happening.
On a new turtle habitat, I suggest that you check the water chemistry levels at least twice a day for the first week until all the numbers are in the ranges we mentioned above for an entire week. Sometimes it may take two or more weeks for the levels to stabilize. You'll need a good-quality aquarium water test kit to do this. Be sure to by the correct kit for the kind of water your turtles will need (fresh water or salt water). The kits are not interchangeable.
Once all the numbers are stable for seven straight days (ammonia zero, nitrites zero or close to zero, and nitrates < 40 ppm), then you can reduce your water-checking frequency to twice a week. But check it more often if the water gets cloudy, turns green, or starts to smell bad; or if your turtles look like the water is bothering them because they're rubbing at their eyes, holding their heads out of the water for a long time, or trying to swim through the glass of the tank or dig a hole in the tank bottom to get out.
Another time that you should check your water chemical levels is if you start getting a lot of algae. Algae's not a bad thing. It's just a plant. In a turtle tank, it helps keep the water clean, and the turtles aren't bothered by it at all. But algae may be a symptom of a problem. Algae uses nutrients in the water that are mainly turtle waste products; so if you have a lot of algae, that usually means that you have a water quality problem. Even with a little algae, be safe and check the chemical levels in the water just to be sure everything is safe for your turtles. Do not use any algaecides! Some of them can be extremely toxic to turtles.