Product must be applied on a very low wind day - windy days will pool the product on downwind side, can suffocate fish. Low winds allow the product to sink in the area of the leak. If area of leak isn't known, full pond treatment is necessary, obviously. Work pond back and forth applying 4 oz product at a time, which covers approximately 10 sq ft. Skimping on product application to save money often backfires and won't solve the issue. These are all things I have learned personally and can relate - I elected not to follow instructions and do it my way, which likely doesn't surprise anyone here. I applied on a windy day, and lost some FHM. I also tried to skimp on application amount, and had to reapply as it didn't address the leak. If instructions are followed and one employs some patience, one can apply to a full pool pond, full of fish, and treat the leak in the amount of time it requires to cover the area of the leak without any mortality. Product is not toxic to fish, but again it can suffocate by pooling on windy day.