Awkward Swimming Fish

[k3rack]

So I cycled my tank and finished about a month ago. I had 3 tester damsels during this and decided to add a goby. Everything is fine and dandy. I took out the 3 damsels and put in 2 perc clowns, a niger trigger and blue tang. The two clowns were swimming on the top of the tank for a while, the tang was swimming normally and the trigger was hiding most of the time and coming out periodically.

Yesterday I noticed my tang skinny and swimming face down into my rock, while the 2 percs were swimming on the edge of the glass sideways. They don't look healthy but have no external indications of disease. The tang shows a half-transparent white circle near it's gill. Not sure if it's sand or a disease.

Anyways, I don't know how to treat the fish or if I should just feed them a little extra and let this pass. The water is not bad: 8.1ph, 0 nitrites, .15 ammonia, 10 nitrate, salt OK, temp 79, nothing weird in the tank.

Besides the percs swimming and the tang spots and swimming, the goby and trigger are totally fine. The goby is happier now than ever and the trigger hasn't changed.

Any ideas on how to overcome this? Any help is GREATLY appreciated since I am new to SW tanks and love my fish!

 

[WILDER]

How many gallons is the tank.
Water changes till ammonia reading is 0.

Does the tang look really thin?
What does it look like when your fish go to the toilet.

I would also post this thread in the marine section of the forum.

 

[k3rack]

It's a 40 gallon. The ammonia is probably 0 since I did a 30% water change since I checked the level 2 days ago. I can check again...

The tang looks really thin, I think I can see bones on it's exterior scales.
I haven't seen that part yet but I can get back.

Thank you!

 

[WILDER]

Being really skinny can mean the fish is being starved, or not eating, fish tb, internal parasites.
Is the spine bending.

Not familar with these type of fish so you might be better of posting in the marine section of the forum.