Yesterday morning I noticed this on my yellow tang. The other 9 fish are not showing any sign of illness. I have captured the tang and she is now in a new quarantine 5 1/2 gallon tank. I only put in a small filter and a heater and something for her to hide in, no substate. Please let me know what you think this is and how to treat it. Also, will I have to treat the whole DT? Thanks in advance!