My son found a snail outside and put it in my fish tank It seems to be doing ok but I don't want to harm the snail Is this ok for the snail?

Well, if you take well care of it...yes. You have to put in lots of plants for him to eat. They also eat fruits and vegetables -it might sound silly, but it's true-, like lettuce and tomattos, bannanas and carrots. Just chop it into a slice, no bigger, no smaller, and put it in there. You need to put lots of dirt and and grass in there, or else there will be a bigger chance of the snail dying. Put a short cup inside and fill it up with enough water so when the snail climbs on it he can reach it and not so much that he can drown if he falls in. Hope I helped!