Last night my husband and I were both bothered by sounds of splashing and of sand being kicked up into the filter. The third time it woke us up, at 2:30am, we decided to turn on the light to see what was going on. (but come on, are these fish or babies?) Everything seemed to be in its place, all kuhlis were accounted for, the filter was functioning properly. One of the kuhlis was looking a little greyed out, but I looked closely and it wasn't fungus, nor was it patchy, so I decided it was probably mood. I noticed it was the one that had two of its black bands partially joined. Then when I got up this morning at 6:45, one of the kuhlis was dead. It was the one I had noted looked a little off-color 4 hours before. The other two were resting in the back of the tank. I tested the water. Ammonia 0, NitrItes 0, NitrAtes 8, pH 7.0.
Other possibly pertinent information.
There was a nitrIte spike when I first added the kuhlis several days ago. It lasted two days but was only high (3ppm

They had been in the pet store for slightly more than a week before I went to pick them up.
There is a diamond tetra in the tank that has popeye in one eye. I believe she injured it while trying to avoid being netted when I moved them into the tank a couple of weeks ago, infection set in, and she still has not been able to recover. I plan on moving her to another tank and treating her, so the kuhlis do not have to go through the medication. (If I can catch her...they are unbelievably fast Another one of the diamond tetras injured his mouth while I was moving them, but he has recovered. They do NOT like being netted!)
I fed them an algae wafer last night, which they did not eat. It was still in there this morning. I also fed one two nights before that, but I suspect the diamond tetras were the ones to eat it. They were very fat.
There were no markings of any kind on the dead kuhli. I have a picture in case anyone is interested. He was laying on his side on the bottom.
Nothing else to note. I am wondering if it could have some connection to the nitrIte spike but I'm wondering why it would happen a few days later rather than while it was high. Or could he have killed himself while splashing around?