Here is an article that explains the reason your fish are gasping for
air. The gills are chemical burned! http://www.vcnet.com/koi_net/finalnet.html
Poisons, Chlorine, Chloramine, and Pollutants By Norm Meck Koi Club
of San Diego 1996

The best way ot prevent this tragedy from ever happening again is to
attach a garden hose chlorine filter to your hose. You can find a
great one for just $32.50 at http://wwww.richdeer3pondsupplies.com.
You should also never no matter how tempting be out of direct site of
a pond or tank that is being refilled!