I believe my city water that I used for my last water change is contaminated. First, some basic info: I've kept goldfish for years, first in Tarrrant County, when i lived at home. I had a fifty-five gallon with a shebunkin and two orandas, all small. Everything went fine for a couple months, did regular water changes, checked water parameters, had double filtration, all fine. Then, one day, I found a fish dead, all others stressed. I treated with Melafix, tried to calm others down, but they just died. One by one, over the course of a day or so. Nothing visibly wrong with them, scales fine, no ich, no tail rot (know what all of these look like from owning a pond). I gave up, took a couple of pond fish, moved to Denton for college. Got a 40gallon tank, sold my 55 because of memories associated with it, and got an Oranda and had two of my pond commons. I did regular water changes, checked levels, treated water with API stress coat. no issues for two years. I've graduated from college, moved back to Tarrant County for work. Now exact same thing has happened, this morning. I've lived in back here for a few months, now I woke up this morning, one of my older commons was dead. Figured it was his time to go, he was a couple years old (I know, goldies live for 20 years, but he's a breeder). Then, a few hours later, the other common died. Neither had any visible signs of illness, scales vibrant, no damage to fins, etc. I went to my trusted fish store, and got a ryukin. My oranda gets VERY lonely and he was depressed without company. So, I got him a little friend. The tank I got him from was healthy, he was healthy. He and the Oranda sat together, then they started swimming together a little bit. thought my oranda was cheering up. Then, an hour later, my new little ryukin was dead. and the oranda was looking worse. I got my oranda in distilled water, in a sick tank, treated him with melafix, ammo lock, etc. Now, he's upright, swimming a little. Long story short, given the evidence, I'm pretty sure it's a contaminent in the water that is causing the issue. My question is, what can I do? I can't afford to fill my 40 gallon with distilled water for every water change. How can I keep my little oranda alive (if I can save him from this?).