I would suggest getting a mud or musk turtle. They are the smallest of the aquatic turtles, only being about 5'' full grown. Also they are relatively hardy turtles and will eat a variety of foods (reptosticks, worms, crickets, etc). They are usually rather timid turtles, but after a while they will get to know its owner and become really friendly.
The basic setup includes:
- 10gallon aquarium (suitable size for at least the first few years, but should upgrade to 20gallon when full grown)
- heat lamp
- uv lighting
- filter
- tank accessories = floating cork, fake or real plants to hide, and some sort of substrate (sand would be ideal).
These guys are fairly common and you can find them at most reptile pet stores. The young ones are really tiny, about the size of a quater. You can find a lot of information on the internet about them. Just use a search engine and look up mud turtles and musk turtles.
Hope this helps.
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Mike
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2 Painted
1 Musk
1 Common Snapper
1 Cooter
1 Map
1 RES
1 Corn Snake
1.1 Water Dragons
2 African Frogs
1 Toad
1.0 Degu
1.2 Chihuahuas
0.1 Cat