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I have a 1 year old turtle who is about the size of my hand in a 35 gal and my friend asked if I could take her turtle because she can no longer care for it. It's about 1 year old but it's only the size of a hatchling? (I don't know why) I said she can bring it over and we will see if they get along or not but I'm still not sure if it's a good idea or not. I don't know any place around me who could take the turtle instead.

The minimum recommended tank size for turtles is 10 gallons per inch of shell length (from top to bottom); depending on the size of your hand, this might be too small of a tank for your current turtle, and you'll definitely need to get a bigger one down the line as it gets older. If you got another turtle, you'd need an even bigger tank if they get along. (Twice the turtles will mean twice the poop and pee, so having a bigger tank and bigger filter will dilute that). A lot of it depends on personality, though, and that is pretty much impossible to predict; it's safe to assume you'll have to separate them eventually.

If your friend can't care for the turtle anymore due to monetary reasons, you'll have to decide whether you have the money, space, and time for another setup and another turtle (with more vet visits, food, etc). If it's not about money (if they're moving or just don't have time/interest), maybe your friend would be open to funding some habitat upgrades, or splitting the cost with you? I'd even ask them to help with future vet visits. It won't be an easy or cheap integration, unfortunately; but if you have the resources to care for the turtle, and you know you'll have them for years, that might be the best thing to do.

It might still be small because it's in an inadequate habitat (no UVB light, too small of a tank, unbalanced diet), but it might also be connected to an illness. Unless there's an irreversible illness, it will probably get bigger very quickly in a proper setup.

I've decided to take it in and isolate it for a few days to be safe. Then if they don't get along I will need to buy extra lamps and filter. I have a extra dock and plants and can use a plastic tub as temporary set up.