redearslider.com

Hello good turtle folk!
I have a male and female turtle approx. 4 years old in a 120 gal indoor tank. My cousin found a huge red ear slider about the size of a mini basketball ( like the kind you win at a carnival).
Can I add him to my tank or should i take him to the local nature preserve, they have a good size stream and alot of sliders there already or what else? I dont want to leave him in the street.
Any advise would be appreciated.
Thanks!

Was their water anywhere near where you found it? If so, you should return it there.
Where are you located? Are RES native to your area? There are lots of issues regarding RES in the wild, since in most places they are considered an invasive species.
To be honest, I'm not sure what to suggest.

I would recommend not putting it with your current turtles. It may have diseases or parasites that would make your domesticated turtles sick. On the flip side, if you do decide to release it back into the wild, your domestic turtles could pass disease to the wild turtle which it would not have an immunity against.

Hopefully someone else will have a good opinion for what you should do.

I'm in the concrete by jungle of orange county California.
And no there wasn't any sources of water near by.

The local nature preserve sounds like a good idea since there are RES already there...

Yeah, nature preserve, that or maybe the turtle is someone's pet who accidentally escaped. Those things can happen.

Maybe post an ad in a newspaper? Or on craig's list? It certainly can't hurt.