Your description sounds like brook which is common on newly added clowns. A picture would help but brook is most likely. You don't have to have a QT you can do the formalin bath and return him to the DT but you need to act very quickly. Brook can kill in a day or two.
I'm going to get the formalin today and start treating...
I have two clowns, 1 is sick, other looks OK - should I treat both or treat the one and just watch the other one closely?
So one died, the other looks fine... I'm pretty sure it is brook
should i treat the one that is still alive? looks healthy, but always is swimming into the current from the PH
Only treat it if you have a sterile quarantine tank to move it to after the formalin bath. If you are going to just put it back in the main tank, the bath would be worthless.
Brooklynella. Formalin bathes. You can use a dedicated bucket, meaning a bucket only used for fish. Be sure that a very small powerhead is used for water circulation. Sometimes you can find 5 gal tanks at the LFS which is perfect such occasions. I like to also use rectangle rubber-maids, which also works very well.