I'll type out what I know and maybe someone will see something I didn't.
I bought 3 chromis from ***** that had not been released into the tank yet. They were floating in their shipping bag to temperature acclimate. Since there were 3 and I wanted 3, I got them.
Brought them home and put them in a bucket with their travel water and ran my accudrip line from the QT tank to their bucket and let it drip for an hour.
I then moved them to the QT tank which had ruck rubble, air stone, and nothing else. Admittedly, nothing else... I forgot to put a heater in it! They didn't really eat. They were a bit confused by the food, but eventually figured it out.
After 60 hours, I found the largest of the 3 dead. He didn't have any external signs of anything, except his mouth was wide open. I'm going to assume he died from poor oxygenation in the water? It was suggested it was aggression from the other 2, but he was completely intact, no nips on the tails or anything, and I never witnessed aggression.
The other 2 finished out QT together, with a heater this time. They took well to the food and seemed overall quite content, except one fish was being aggressive towards the other some. At one point the bullied one had some patches of missing scales.
When QT was over, I moved them to their new home. They both were doing pretty well, except the bullied one seemed to like to hide a lot. I figured it would take time to feel safe. He would still come out for feedings, but then I started seeing a lot of signs of aggression. The aggressive one would attack him when he came out for food, and chase him around the tank. It got to where he wouldn't come out until the other one seemed to get his fill of food and then go "nap" in his cave. But if the aggressive one saw him out he'd chase him. He started fighting back a little, but then one morning he disappeared. He didn't come out for food and I couldn't find him anywhere. I then found my crab hiding in his cave chewing on a fish head. So not sure how he died, but he was definitely dead.
Then the last one was fine, eating lots, even ate some frozen mysis shrimp that were half his size! He was very active and never hid when you came up to the tank.
A couple days ago I did a water change and he hid the whole time. It was his first water change since being in the new tank. I also rearranged the equipment some.
Yesterday I added a surface skimmer and new filter pad to the HOB filter. The fish was still hiding the whole time and didn't come out when I put food in the tank, but I could see him swimming in his little cave.
Then last night as I was about to go to bed, I decided to check the tank with a flashlight. Nothing was going on in the dark, but then suddenly the fish appeared in my view, which was weird. I then realized he was kind of being blown around and I got a bit scared. I fished him out, which was extremely easy, and put him in my HOB refugium with an airstone on low so he didn't have to fight the current and would have ample oxygen.
I observed him and he was struggling even with little current to stay right. Too high, too low, on his side, upside down, he couldn't get it right. He finally wedged himself in a spot to hold him still and seemed to rest. I thought he was dead, but he was still breathing. He was still breathing this morning until about 8am when he finally let go.
His body was fully intact, I saw no external signs of anything. His coloring was still good. His gills were salmon colored, his mouth closed, but I opened it and didn't see anything strange. His body was smooth, no bumps.
I don't know much about all the diseases out there, just the common ones.
My water parameters were 1.024, pH 8.1, ammonia 0, nitrites 0, nitrates 5, phosphate 0. I tested ammonia, nitrites, and nitrates again this morning after his death and got the same results.
