I like turtles, but I don't know a lot about them. I'd love to have one as a pet, but I want to do my research first, so I can give my pet the best home possible.
I have an old fish tank I could repurpose for a turtle, but it is pretty small (I think at most it might be 10 gallons). Is there any sort of turtle that would be happy in that space, or am I going to have to upgrade for maximum turtle happiness?
10 gallons will only be sufficient for baby turtles. One rule of thumb I see is 15 gallons per inch of turtle length, though I think 10 gallons per inch could be a feasible minimum. This would mean that a full-grown red-eared slider (using this as a typical turtle) would need about 75-125 gallons, though more would be better. You could probably get by with a 40 gallon breeder tank for a few years when it's young, but would need to eventually upgrade.
There are smaller turtles, such as musk turtles, which could actually do well in a 20-30 gallon long tank (they like it shallow). They also have slightly easier maintenance (don't need as strong a basking spot). They don't usually have as distinctive of patterns as other turtles, but they still can be pretty neat -- their long necks and small plastrons make them nimble little climbers. I'd probably recommend one as a starter turtle.
There's also non-fishtank options, such as turtle tubs (either pre-made, or built yourself) that are just wide tubs with sides just tall enough to keep them from climbing out. These can sometimes be cheaper to acquire than 90 gallon aquariums, but take a lot more floor space.
Whatever way you go, you'll need to invest in a good quality, big filter as well. They're very messy, and constantly cleaning an underpowered filter will get old really quickly.
This page seems to have a lot of good stuff too.
There are padlopers... There's a species that only gets to 1.5" and it's largest species only gets to 3", they just don't do as well in captivity as a mud turtle which gets only to 3-4". If you can adequately meet the dietary needs of the padlopers this should be okay, as padlopers don't do well in captivity DUE to their diet, not environment. Mud turtles are known to thrive in captivity reaching and exceeding 40+ years old. Not sure if you have a standard 10gallon, but there are breeder style 10gallons that are lower and wider than the typical 10g high (24"x9"x13" vs. 20"x11"x 13"). The mud turtle would be OK until it becomes a juvenile but would need to move to something larger at that time. As stated above by @dougkavendek most turtles don't do well in tiny spaces, but the mud turtle may be an exception.
