Miniature Turtles May Not Always Stay That Way
If you purchase a miniature turtle for a pet, there is a distinct possibility it may not stay that way. A very small turtle is most likely a juvenile, and will grow. How large it will grow depends of course on the species. So it's somewhat a matter of buyer beware.
It's not that there are no small turtles. A small turtle is best for a pet, at least from the standpoint of finding a suitable enclosure. Most small turtles won't grow larger than 5 or 6 inches long, some 8 or 9 inches. These are nice sized turtles but you really can't call them miniature turtles. When you buy what appears to be, or is advertised as, a miniature, be advised that it is going to grow, and you are going to have a much larger turtle to care for than you may have counted on. This can be an added expense, and unfortunately some people never get around to treating their "miniature" like a larger turtle should be treated, and the animal eventually sickens and dies. Or it is simply discarded.
Many of the turtles that are marketed as miniature turtles are baby red-eared sliders. These are not large turtles, but still grow to an average a foot in length. If you want something a little more "miniature" there are three species worth considering. All three will stay smaller than 5 inches in length, so you won't have to build a huge enclosure, just something that gives a 5 inch turtle enough room. Get a male if possible, if only for the reason that when fully grown, they are normally smaller than the females.
The Reeve's turtle makes a nice pet in that it is more active during the day than some of the other more popular species. As it is an omnivore, it enjoys a variety of foods; everything from insects, to chopped liver, to fruit, and in that sense is an easy keeper. Some Reeve's turtles have been known to develop quite interesting, some would say quirky, personalities. A semi-aquatic turtle, it needs a place to swim, dry land to roam around on, and also enjoys basking under a heat lamp. A thirty gallon aquarium tank will give the Reeve's turtle plenty or room, a 40 gallon tank is better if you have two of these turtles. While the species is cold hardy, they will be at there best when in an environment approximating room temperature.
Another possibility is the Musk turtle. Musk turtles, along with mud turtles, are the closest you will get to owning miniature turtles. A typical musk turtle may attain a length of 5 inches, but 3 inches is fairly common. Again, getting a male turtle will give you a smaller turtle. We usually think of the giant tortoises as being long lived, and they are, but a little musk turtle can easily reach an age of 50 years. If you keep good care of your pet you can count on having it for a good long time. Musk turtles spend more time in the water than does the Reeve's turtle and is a bottom walker. It still needs an area of dry land and a heated place for basking. Water condition is very important for this turtle, and a filtration system is a must. While an omnivore, it eats primarily other animal life, but it will do well on many commercial turtle foods. One drawback the musk turtle has is, if disturbed, in can secrete a foul-smelling fluid, earning it the name, "stinkpot turtle".
A third possibility is the Diamondback terrapin. Yes, that's the famous mascot of the University of Maryland. This turtle is found in brackish waters along the east coast. Its coloration varies from grey to yellow, and from brown to white, but all members of this species have a diamond pattern on the shell. Males generally do not grow more than 5 inches in length, but the females can grow to almost twice the length of the males. While the terrapin spends much of its time in the water, it nests on dry land, and prefers an environment similar to the two species previously mentioned.
If you're willing to let go of the idea of owning miniature turtles, and think small turtles instead, you should be quite happy having any of the three species mentioned above as pets.