Ahem, ribit. Anyone know much about frogs out there?

Last May I put 3 large tadpoles from the pet store into my 400 gallon
pond, here in San Francisco. They are rana pipiens, and I was told
they'd develop into frogs by August. Well, here in early November,
they look pretty much like large tadpoles still. One has larger (rear)
legs, but I don't see any development of front legs.

Last weekend I went to a different pet store to replenish my supply of
elodea plants, which the local raccoons think is a delicacy. There
were small frogs there (from Japan I was told) and I purchased 2, let
them go, and they've quickly disappeared, which I expected to happen.
Anyway, the owner of this pet store says that tadpoles won't develop
into frogs unless the water level gets low, and they're forced to.

Has anyone ever heard of that?

Thanks!