We have a 3000 gallon pond, three feet deep, wintered with small airator and a heater to keep open water, our fish survived well but the frogs all seam to have died. A warm spell about a month ago had several frogs out sunning themselves near waters edge but now lots of frogs are turning belly up on the bottom and when we scoop them out they are soft and rotting. We do not see any live frogs now that weather is better. Last year also had many many dead frogs on bottom when weather warmed up. Why would the frogs die now when they seem to have survived most of the winter?