Today I shockingly found one of my Zebra Danios attached to one of the filter intakes on my tank, dead. I'm totally clueless on what happened. I tested the water immediately and nothing out of the ordinary - Ammonia was 0, Nitrites were 0, and Nitrates were around 20. I did a physical examination of the little guy and he looked to be fine. Then I decided to examine the whole tank and all the fish and I everything looked find until I noticed another Zebra, still alive, but not moving a whole lot staying around the surface. On his left side there were red marks, couldn't really find too much info on the internet. So I set up a hospital tank with a 10 gallon, used some water from the main tank, a spare filter, and a cup full of gravel and put him in there. He barely seemed to be able to swim, he would just sink to the bottom and not move much. I had to go out for a bit, and when I returned to check on him a few hours later, he was on the filter intake.
I took a picture of him after he departed and you can notice the red marks on his side. His other side was flawless, his gills looked fine, nothing wrong with his mouth or eyes, just those red markings. What is this? My other Zebra did not have these red markings but died. Just hope whatever it is won't affect any of the other fish in the tank.
I am treating the main tank with Melafix, but I don't think that would have anything to do with it.