I have two tanks, a 20 gal, and a 55 gal, I did a 50% water change on the 55 put the conditioners in and everything. Let it sit for a bit to let any chlorine dissapate. And put the fish back in. And keep in mind this is an established tank that's been running a year. I frequently check the water levels with the master test kit and every thing comes back fine. It had 4 VERY small koi and 2 gold fish in it. But i have a 75 gallon filter on it so it was fine. But a few hours after I put them back in after the water change they all died! I tested the water again, readings came back good and there was nothing sketchy about them. I got another heater, put it in. Water got up to Temps and I put some feeder guppies in. Only 4, and they are all now dead. Only thing left to do is to take all the water out, scrub it and put new water in....any ideas why my fish died? Or any suggestions?