Pearl Gourami's colors fading and getting thin

[HLL]

Hello Everyone, 

I have 1 pearl gourami, 1 male dwarf gourami, and 2 male gold gouramis in a 36 gallon tank for over a year.

The pearl gourami has given me some concern because he has been getting thin, and his colors have been fading. He is still eating but slower than before. There has been no sign of fellow gouramis bullying him. Is it possible that he might have internal parasites or bacterial infection?  

All the gouramis have not been through any medication before since I got them. If there is a possible disease, should I treat the whole tank or isolate the sick one? Should all of them be medicated to ensure they are clean from possible parasites or bacterial infections to prevent future issues?

If I need to treat them with medication I am planning on using API General Cure or API E.M. Erythromycin. 

Water Parameters using Terta Ammonia & 6-in-1 Test Strips

Ammonia: 0

Nitrite: 0

Nitratre: <20

PH: 8.4

GH: 25

Chlorine: 0

KH: 300

Water Temp: 76 

Thank you in advance for your help and advice!

Edited January 27, 2021 by HLL

[JettsPapa]

Hello,

I'm afraid my best recommendation would be to get him away from the other gouramis, and if possible give him a couple of female pearls for company.  I know you said you haven't seen any signs of bullying, but with a dwarf and two male gold gouramis in the tank with him I'm surprised there hasn't been a blood bath.  Is it possible they're just now reaching maturity?  If yes the situation will likely only deteriorate with time.

It's possible he's sick or has parasites, but I suspect his biggest problem is stress.

[Colu]

With symptoms he is show I would be suspicious of internal parasites i would treat with general cure

[Mr_Manifesto]

I would isolate this fish in a quarantine tank and observe.  It could be stress and/or internal parasites.  My experience with Gouramis is they are rough with each other.   I've kept two groups of Gold Gouramis, (2-3 each) in two seperate 55 gallon tanks, and at some point, they always start fighting each other and eventually one would die off.   I would guess that pearl gourami is being muscled out by the others.  If you can't isolate this fish, you can try medicating the tank, but if it is being bullied, it will die unless you rehome it.  Fish are very quick about it,  they will just chase one away, and the others will see it as being weaker and start doing the same.  It might not even be one fish, it might be each one doing it and the toll is only on the Pearl.  You may never notice it unless you look at the tank constantly. 
Isolating the fish would be the best option in my opinion,  If it is sick, you reduce the risk of infecting the others,  and if it is being bullied, you remove the fish from the threat.

Edited January 28, 2021 by Mr_Manifesto