It's beginning to sound like something in the tank - perhaps the plant. It has been known for a staff member to spot snails on a plant and to treat them with terrestrial snail killer, which is not very good for fish.
Is any aerosol used in the room with this tank - deodorant, air freshener etc. Or scented candles, plug in air fresheners? Paint fumes? Or anything else which could have got into the air and then into the tank?
I am definitely with you that it is something in the tank. I just wish I knew what. No candles, air fresheners etc. I do think it was either the plant, or the plant holder (will NOT make that mistake again). The plant did have some black spots on it, like rot, and now that I got the cory into the hospital tank, I've completely dumped the contents of the old tank. Luckily I have multiple Fluval filter pumps, will buy new sand, put in new charcoal, new biomax, and have boiled the fish barrel decoration and also thoroughly rinsed the inside of the tank, and wiped it down with a clean cloth and will let it air dry.
I always seem to have these weird issues, and it could have been so many things, and I think because it's not common, there isn't a normal answer, like the mistakes most people make (which I did), so I'll never actually know what the cause was, since my measurements were all fairly decent. Still no idea why the white murky cloudiness, and the algae that wouldn't die. Ugh, now time to start cycling again and hopefully the cory will live since the hospital tank was only up for two days...
Oh, when I took out the filter, it had some weird brown globules, looked and felt like nasty boogers. Wasn't green, but was slimy. Maybe that was it?
