This happened to my friend a while back and I was wondering if anybody could answer it. To sum it up, he had a 40G Fresh with a (I believe) Cascade Hang on filter, standard strip light and a heater just to keep it balanced in the winter. He had some black moors 1 butterfly koi (I know, I know). Anyways, all the fish were doing fine one morning (and were for a while) and then sometime in the afternoon the koi started getting real sluggish. Within about 2 hours time, the tank went from crystal clear to the koi and a lot of the gravel being covered in what looked like some sort of white powder. Almost like grated parmesan cheese. About 5 min. after that, it was on it's back. SOOOO...he drained the tank, cleaned everything and filled it back up. All he had in there were 2 black moors. About 2 months later (give or take) the same exact thing happened except the Moors were effected. They kicked the bucket and he gave up on fish all together. As for readings (ammonia, nitrate, etc) I wouldn't be ablt to tell you. But from what I've told you, can anybody think of what could've happened?
No. NOTHING was put into the tank.