I had started with 2 feeder goldfish in a small aquaponics set up. I had them for about 4 years when the pump died and I decided to dismantle the system - the fish are now about 5-6 inches or so in length, plus tail. I moved them to my kindergaten class, to a smaller tank that hadn't been running for a while, so essentially a new set up. No filter, just an air pump, and small water changes every week. They were quite happy for about a month. (I realize the tank is too small, they were destined for my pond if it ever warms up...) Then, I came to work last Monday morning, and the tank was completely sludgy- I couldn't see more than an inch or two into the water, and there was a thick layer of foam on top. The larger fish was dead (possibly coincidentally, both finches next door were also dead...). I.ve been doing 50% water changes nearly daily, and it is mostly clear again. The surviving fish was swimming some early last week, then huddled in the bottom corner last Thursday and Friday. I expected to find him floating yesterday, but he is much more responsive, swimming some and eating some. He still seems twitchy at times, and often is on a tilt so,his tail end is higher than his head end. Last Monday when the tank first was sludgy both he and the dead one were mostly black (they were both typical orange with lighter bellies), and now he is darker still. Any ideas? It's really weird! The only thing I can find is ammonia burn, but it seems they should darken in recovery, not during the bad phase? The part that stresses me out especially is the dead finches the same day... Any chance it is from something airborne?