fish dying

Question
I have a 1500 gallon pond with a tetra pond filter, one water fall and another rock/water accent. The pond is about two years old. My koi, and two gold fish (about 9" long) out of the blue had died along with a bunch of fan tailed guppies. I have three angel fish and three tiny talapia, which are all fine. The talapia I got about a week before my kill off. Right when the fish started to act weird there was a huge hatching of tadpoles. The PH and the ammonia levels are fine. But the koi and goldfish were gasping for air like the PH was to high or to low. Will a million tadpoles have an affect on the fish.

Answer
Yes, all those tadpoles would put a strain on available oxygen in the water.  Once they are frogs (or toads), they will introduce loads of ammonia, that is if they stay in the pond after hatching.  If they all died in one day, I would suspect some sort chemistry issue.  If the population dwindled over the course of say, two days, it is most likely due to some illness, perhaps a bacterial infection.  Oxygen issues can show varied results and are harder to diagnose.  The fish's behavior leads me to believe it is an oxygen issue, but some compounds such as ammonia can cause the fish to demonstrate the same behavior.