I believe activated carbon will help absorb toxins in the water. You might give that a try. I'm afraid I don't know what the proper gallon-ration would be, but for a large pond it would probably be quite a bit.

Like nancy said...get some activated charcoal in the filter. Also get some aeration going since gasping at the waterfall is a sign of lack of oxygen. It could be that the water actually is low on oxygen or it could be that their gills are damaged. At any rate, an aerator will help a lot. Also I'd salt the water to 0.3%. This is always a good thing to do when fish are sick. To get to 0.3%, add 2.5 pounds of salt per 100 gallons. Don't do at all at one time....add it over a 2 day period.