Frogs in a pond / dam is bad news....frogs always return to the pond where they grew up, when its matting time...so 1 or 2 frogs laid their eggs in your pond now....next year you will have 100.
Koi dont eat tadpoles as they have a bitter taste, they might try it once but will spit it out almost immedialty.
There was a topic on one of the 2 SA forums (or was it the KOISA) where the frogs where trying to mate with they guys koi in his pond an the frogs where actually sufficating the koi with their deadly love embrase.