Since they're wild frogs and it's summer, I'd leave them be if you don't need the fountain to run. Raising tadpoles in tanks is a lot of work, don't do that to yourself.

Just to be safe I wouldn't unclog the filter just yet. Rather than trying to change all the water, I'd introduce manageable amounts of new water, maybe a few gallons at a time. (Unless someone else has a better suggestion.) Maybe someone can find the average amount of time these tads take to grow to frogs, so you'll know about when it should be safe to get the filter going. (If the filter has some kind of cover or mesh that's smaller than the tadpoles, it might be safe to run.)