I have had a goldfish pond in my back yard since 1978. Every couple of years I drain it and clean out the old layer of oak leaves and muck and refill with fresh water. My fish were healthy and I changed the water like I normally do. I usually siphon existing pond water into a child wading pool and put in a aerator pump and move the fish to the pool and the fish seemed happy after the move. After cleaning the pond I treated with a pond anti-chlorine and water stabilizer product that I purchased in the pet store for fish ponds. I then floated the fish in plastic bags until the water temp was the same in the bags as the pond (40 minutes). I released them back into the cleaned pond and within a few hours they seemed sick and lethargic. Two days later I found 8 of my fish dead in the pond and the remaining 10 not eating and lethargic. I have done this many times the same way using the same wading pool, etc and the fish always seemed happy with their clean pond. The only difference I know was I used a different pond anti-chlorine product that I had purchased a couple of years ago. Does anyone know what could have caused this? Do anti-chlorine products go bad after a couple of years of storage in the basement?