You probably got nematodes from leaves or some plant possibly or if your soil is from outside it could be that too. Those frogs are hardy so I wouldn't consider that an issue if it's not for a long period of time. Although the frog would probably adapt and adjust to the warm temps. As for the medicine I would never use a credit card just because I'd be worried of breaking the jaw of the frog. What I did was inject crickets when my Leopard Frog was sick and he ate the cricket and got all of the medicine. You can do this for whatever the frog eats most. Like my toads favorite food is a worm or waxworm. I'd inject it to one of them even a waxworm. Even though it's a fat food if the frog will take it then that's the most important thing. I think there are beneficial bugs you can release to eat the nematodes but don't quote me. You would have to strip down the tank and give new dirt to assure they are all gone. Hope this helps.
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