@jayOne thing I notice about your tank is that there are a lot of fish and the water always looks cloudy. I’m not sure if that is a direct cause of this issue, but it is something visually different than from what I’d like to see in quarantine tank.
Are you managing ammonia only through water changes?
Remember that copper only manages external protozoan diseases. There are some internal diseases that are tough to diagnose and impossible to treat.
I once had a west African angel doing great in quarantine. It was a rare fish, so I did a photo shoot with it. I used a flash, but I was careful not to shock the fish too much. An hour after the photos, I found the fish twitching and unable to swim. The next day it was dead. Upon necropsy, I discovered it had an internal nematode infection.
working more the internal route on the fish
had an Achilles now pass away with a bit same symptoms.
Fish actualy died 2 minutes before I decided to check some internal things...never opened up a death aquarium fish like this.
Decided mainly to try and empty stomach/intestine area and have a look what came out? Dont know how things supposed to look but does bot look very natural....wonder if you can elaborate.....
also made a little video..... see below
also tried to take some pictures of gils...again is this normal this damage on the gils? not tiuched just opened the area....
can you please reply your thoughts
