Massive Die-Off

So we went out of town for a few days and came back to a total mess. Lost at least half my stock. Tested for ammonia/nitrate/nitrite and everything looked good. Temperature was good. Did two big water changes and no one else has died yet.

Only thing I can think of is I added some neons without QT about a week before. Did I introduce some sort of disease? No signs of anything that I can see.

May have figured out my own problem. Was thinking things through and realized I did the water quality test after my first WC. Just a few minutes ago, trying to investigate further I discovered a large turquoise rainbow who had died and was tucked in a little cave under a piece of driftwood. I knew he was gone but I thought perhaps he had been eaten up. Had decomposed a good bit and was STINKING when I brought him out. He may have caused some serious quality issues that I didn't pick up on because of the water change.

Indeed. Did about 60% first round, and 70% second. Will probably do another tomorrow.