They're pond snails. They might eat decomposing leaves, sinking feeder pellets, possibly a bit of algae, and generally will survive a nuclear holocaust. Many people find them obnoxious. Others have no problem with them. Either way, they won't damage your plants or harm fish.
You'll end up with pond snails in your tank no matter what (they, and their egg sacs, hitchhike in on plants). You can jumpstart the process by putting these guys in there, or do something else with them. Up to you.