Clownfish poop

[Alyssa]

Hello,
I am in a bit of crisis. I have a 90 gallon tank. Phosphates are a little high but all other parameters are good. I have a purple tang, 2 neon gobies, 2 clowns, shrimp (x3), snails and crabs. Diet consists of frozen mysis, pellets and flake with occasional brine as a treat. About 4 days ago I noticed that he has lost his appetite and has a piece of long, white, stringy poop hanging from him. I have bought 3 different medications: Medic by PolypLab, PraziPro and Metronidazole. I have been dosing the food in the DT with the Metronidazole for 3 days. Only problem is, the clown will not eat! I have a QT and my question is should I quarantine him or not? What type of medication should I be using, can I switch now?