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Ok, I love my turtles very much but I always feel sorry for them having to live in a tank. I'd love to build them a pond to live in but unfortunately I don't have a yard

This is just an idea...

There is an island near where I live that is INFESTED with turtles that look like RES though I am not positive if they are because their shells are darker in colour and I'm dreaming of letting them get their freedom on that island.

No joke, the last time I visited the island there were literally hundreds of thousands of them everywhere basking, in ditches, ponds, marshy areas.

Would they be happy if I let them free or would they be helpless in the wild having lived 7-8 years in tanks?

if its already overcrowding then hell no

Helpless for sure. You should see my turtle trying to catch prey after only a year in a tank. Its pathetic. Plus by being in a tank for that long they haven't been exposed to wild turtle diseases, and are therefore more susceptible to infections.