One of the most frequent questions I'm asked is whether you can house more than one turtle in the same tank. The best answer I can give is "sometimes." I've tried it, sometimes successfully, sometimes not. In fact, the two turtles that started this site in 2010 got along for about five years before they started fighting and I had to separate them. That was kind of a record. Most turtles decide whether they like each other or not much sooner than that.
The problem is that in nature, turtles are basically solitary animals. Groups of turtles may live in the same place because the conditions there are favorable for them, but they aren't really "social" animals. They don't have a real social structure nor any division of labor that would qualify them as being "social animals." They're just solitary animals who happen to live in the same place because it has the things that they need.
In captivity, turtles differ in how well they get along. Juvenile turtles usually get along well enough and may even seem "friendly" toward each other while they're little. But as they grow up and the turtle equivalent of puberty kicks in, turtles often start fighting (especially the males if you have more than one male in the same tank). On the other hand, some turtles get along for their entire lives. They're very much individuals in this regard.
When turtles in a tank start fighting each other, there's no solution other than to separate them. If you don't, the chances are that one will seriously injure the other. That's why this site has sometimes had one turtle and other times has had two or three turtles since I first put it up. Turtles who got along just find when they were little often stop getting along once they grow up. When that happens, I have to separate them into separate habitats; and for cost and bandwidth reasons, only one of the habitats is set up with a video feed.
Here's a video of the most recent time that I had to separate two turtles. It's a little choppy because it had to be retrieved from cache and reprocessed. The story is that these two male turtles are separated in age by several months and had been getting along find for about a year. But when the older turtle hit "puberty," it started resenting and harassing the younger one. Here's what happened.
You can see the younger turtle on the left trying to keep away from the older one, hanging from the thermometer sensor with his tail tucked in, probably because the older turtle had nipped at it. He still had his tail when I checked, but I guess he wasn't taking any chances. The older turtle wasn't satisfied that the younger one was staying on the other side of the tank, however, and continued to harass him. When he bit the younger turtle's neck, I separated them and moved the younger turtle to another habitat.
This is an example of territorial aggression. In nature, the younger turtle would simply swim far enough away from the older turtle that there would no longer be any conflict. But in captivity he couldn't do that, so I had to do it for him by removing him from the tank.
Sometimes what seems like fighting is actually mating behavior. It takes some experience to tell the difference, and even then, it's still guesswork to some extent. Here's what to look for.
Before mating, as part of a courtship ritual, males often nip at females' necks, feet, and sometimes their tails to see if they can get the females interested in romance. If the females aren't interested, they'll turn their shells to the males and sometimes snarl at them, and the males usually leave them alone for a while. The nipping is gentle and playful and doesn't actually injure the females in any way. When the female objects, the male leaves her alone.
Fighting behavior is more like bullying and has to do with territoriality. The more dominant turtle doesn't want the less-dominant turtle sharing its space, so it tries to chase it away. Territorial aggression may occur in the water, the basking area, or both. It usually starts with the more-dominant turtle fluttering its hands at the less-dominant turtle, who may flutter back or swim away (or flee the basking area if that's where it occurs). The dominant turtle may also continue chasing the less-dominant turtle away even after it has given up the fight.
If the show of aggression progresses to actual bite, the bites will be more forceful than the harmless nipping of a mating ritual and may actually cause injuries. When this happens, or when it seems inevitable that it will, you will need to separate your turtles before one of them seriously hurts the other.
When keeping multiple turtles in one habitat, you have to keep in mind that most turtle fights are about territoriality; and territoriality is about resources. For turtles, this means sufficient amounts of space, water, light, and food. You want to make sure that there are sufficient resources that none of the turtles feels the need to compete for them. If there is scarcity in any of the resources, then fights are almost inevitable. Here are some things you can do to reduce the chances of that happening.
Here are some things that you can do to reduce the chances that turtles in captivity will fight. None of them are guarantees that it won't happen, though.
Make sure your turtle's habitat is big enough. At a minimum, you need to have at least 10 gallons of water for every inch of your first turtle's carapace length, plus half again as much space for each additional turtle. In other words, you should have at least 10 gallons of water for each inch of the first turtle's length, plus another five gallons of water for each inch of every additional turtle's length. That's the absolute minimum. More is better.
If you obtain your turtles when they are juveniles, you should size the tank based on their expected adult sizes unless you plan to upgrade the tanks as they grow and are confident that you will be able to actually do so when the time comes. If money is tight, you probably should keep just one turtle.
Use visual barriers. When a dominant turtle chases a less-dominant turtle away, one of the things it's saying is "Get out of my sight." Plants, rocks, and other visual barriers inside the tank may reduce fighting by allowing the turtles to stay out of each other's sight.
Try feeding your turtles on a daily basis. Most keepers feed their turtles every other day. Some turtle aggression problems have been solved by feeding them on a daily basis or even twice-daily, presumably because it reduces food-related territoriality. Of course, if you decide to try this, feed your turtles less food at every feeding. You can read more about turtle feeding here.
Provide large or separate basking areas. If your turtles' fighting tends to occur on the basking area (or if one dominant turtle is preventing the others from basking by chasing them off the platform), you may need to buy or build separate basking platforms or one huge basking platform with visual barriers. You'll also need separate basking lamps for each area, and possibly separate UVB lamps if one lamp can't adequately light the entire basking area.
Be especially careful about water quality. Anything that causes stress to your turtles will increase the chances that they will fight, and this includes water quality. If you have more than one turtle in your tank, you will have to use a larger filter and probably do more frequent water changes.
Try to avoid housing multiple male turtles in the same tank. This can be tricky if you get your turtles when they're juveniles because it's almost impossible to tell males from females when they're little. Once they grow up, male turtles are much more likely to fight than females are and will probably need to be separated.
Be careful about mixing sizes and ages. I had one case where a fully-grown turtle tried to bite the head off a newly-introduced yearling. Obviously, I had to separate them. On the other hand, some adult turtles will tolerate younger turtles, but then will turn on them when they start to mature.
Consider different pairings. Sometimes a turtle will especially dislike one particular turtle, but will be able to get along with a different turtle. If you have multiple habitats, it may be worth seeing if you can move your turtles around until everyone is happy with their tank-mates.
Finally, bear in mind that turtles don't need the company of other turtles to be happy. In fact, most turtles prefer not having other turtles around. If your only reason for wanting to have two or more turtles is so that your one turtle "won't be lonely," don't worry about that. Pet turtles don't need company. Your turtle will more likely resent having another turtle in the tank than be happy about having a new friend.