Hi all. I have a pond which had been completely neglected since it was installed by the previous owners of the house. It is 12 ft x 10 ft by about 2.5 ft at its deepest point and has a black flexible pond liner. I never believed there were any fish in it until we saw one recently very briefly between all the weeds and brambles overgrowing the pond. I decided to clear it out and try to restore it. I should add that I have another pond in the garden (12ft x 6 and about 18 in deep with about 15 fish in it averaging about 5-6 inches. Having cleared out the overgrown pond, I added a 1500 lph solar pump and some more oxygenating plants (hornwort, parrots feather canadian pond weed etc) I decided to add some fish having checked the basic water parameters. I used an NT labs kit to check Ammonia, nitrite and pH. pH is about 7.8, Ammonia is 0.05 ppm and nitrite is about 0.1. I added the fish about 4-6 weeks ago and they seemed to do very well. A couple of them grew very quickly. I got up to about 12 fish in the pond and was really starting to enjoy it but the one area which was bugging me was the clarity of the water which was a bit cloudy. Having looked at it though a glass, i saw the water had sediment suspended in it, so I decided to treat the water with TetraPond Crystal Water. I followed the dosage recommendations but was horrified to see the fish seemingly gasping for air at the surface about two hours after the product had been added. I managed to transfer some fish to the other pond a couple of which survived. The rest (10 fish) have died. They were Blue and Golden Orfes, small Koi, Ghost Koi and Shubunkin between 4 and 10 inches. I went on the internet to research the process by which the tetrapond product works. It's called flocculation, but the product claims to be completely safe for fish and plants. It seems to me that the process used up the oxygen in the water which killed the fish. I have tried to contact Tetra but they have failed to reply despite a promise to reply within 24 hours. That was a week ago. With hindsight I think their product imformation has insufficient detail about the risks associated with it's use, for example maybe I should have used it at a time of the day when there was more oxygen in the water. Has anyone heard of anything similar? What did I do wrong? Grateful for any help. I'm now nervous to buy any more fish until I fully understand what went wrong.