sick damsel

Hi all, this yellowtail damsel is two days new into the tank. Initially, he swam around fine, and then found a hiding spot in a rock. But today, I found him on the sand bed on his side, gasping for air. I don’t have an established quarantine tank, so I put a strainer at the top of the tank and...

I got to act pretty fast here. So I have 8 Fiji blue devils in a 10 gallon QT. One (the one in the video) has bruising pretty bad but all the others seem to be alright. How do I treat this guy for the best outcome? Second day in QT I used 50% display tank water and the rest new. Salinity is at...