Dying Fishes

The goldfishes and mollys are dying in the pond.  I wonder what is happening.  The outdoor pond is only using rainwater.  So is the rainwater getting toxic for the fishes?  I wonder if it is the pollution or maybe due to regular fogging which may leave residue toxins into the pond.  Luckily, the arowana is strong and have a steady diet of dead goldfishes.  I felt terrible with my fishes passing.  I wonder what I can do.  The pond has an oversize filter with bio-treatment.  Maybe it is time to clear the filters.