Clownfish slow and discolored

[Coachb9]

So I am fairly new to the hobby. I have had these 2 clownfish for a month or so. In the last 2 days, one of them starting acting lethargic and not eating. I noticed today that he was starting to discolor. The clown is also breathing very heavy.
I down know what may have caused this. The other clown is acting normal and eating, swimming etc.
I have a 20 gallon but having a 55 gallon that is currently in the cycling process. I do not have a QT. I don't want to jeopardize the other fish if this clown isn't going to make it. The 55 gallon still has an ammonia reading of 0.50 so I don't know if I should rush and put other fish in new tank and leave just clown in 20 gallon and try to treat him alone.
Other fish in the 20 gallon is a small yellow tang and a small blue spotted puffer.
Any feedback would be appreciated. Picture attached

 

[snorvich]

Picture not definitive. From your description and timeline, sounds like brook or velvet.

 

[Coachb9]

Ich treatment

Ich treatment

Would a basic ich treatment suffice to get rid of either of these diseases?
Should I treat entire tank?

 

[snorvich]

Would a basic ich treatment suffice to get rid of either of these diseases? No
Should I treat entire tank?

all fish should be treated

 

[Coachb9]

Woke up this morning and the clownfish was laying flat on the substrate with hermit crabs poking at him. I will keep my eye on the rest of the fish and treat the tank. Hopefully I can get it taken care of.