Mostly over the 10 years I've had a pond, the frogs seem to come and go on their own. I too have bought tadpoles what were ready to morph hoping they will stay but I never see them after that. One summer I had a male, female and small one that I called their child, Frieda, Freddie, and Fritz. But come the next spring, they floated to the top dead. I had a huge bullfrog in the back preform in the spring. He loved it because it was totally mucky. But we emptied it, cleaned it, and put fresh water back in. We put him in. He stayed maybe 2 days and is gone now. Then in the front, yesterday I found a female by the little pond. Wonder if she stayed..