Frogs travel long distances at night during rain events. Presence of fish in the pool is an easy answer. Someone put them in; probably kids. As I said before every fisherman wants his own fishing hole even if it has to be a temorarily abandoned pool. Food source (zooplankton)for all these fish came in with the fish transport water. Newly hatched tadpoles were also no doubt eaten by adult "bream" FYI. Pool chlorine very quickly disipates when it is not frequently renewed.
PS. Many many people call young game fish "minnows". All the minnows in the pool were probably young fingerling bream or sunfish; very probably green sunfish.
