Help urgently needed

[FishLover85]

Hey, I really need some help off anyone who is able to help!
I have an orange tail damsel, and its very sick, it looks like he has scales missing and today when I woke up I noticed his fins look frayed now!
We have in the tank a humbug damsel and 3 clownfish, all added at the same time (the aquarium never told us about compatibility) but they have all been sharing the tank problem free since August. We had a Tang but it died a few days ago, it also looked like it had scales missing, when we went to the aquarium they told us to use Vertones antiprotozoal, but it didnt work for the Tang and I'm afraid its not working for the orange tail damsel either
The humbug damsel is now picking on the orange tail, and not letting him in any of the rocks, or ornaments and not letting him eat either
I'm pretty sure he will die but I don't want the others to get sick either so does anyone know what is wrong with my fish and how I can fix it so they don't all die one by one???
The humbug never picked on the tang either so he isnt bullying them one by one the humbug was protective of the tang and never let any other fish pick on it
If anyone can lead me to the right direction so I don't have all my fish get sick and die I would really really appreciate it!
Thanks

[Judge]

Sorry Fish Lover but you have not provided enough details for us to help you. We need info like how big is the tank, how long has it been up, what filtration, water source, and most important....water params. Please don't tell us you took a sample in and were told "they all look good". LFS are notorious for saying that but it provides no info. Was the tank cycled? I am sure I have forgotten some questions and I am sure others will swim in and help too but please get back ASAP so we can help.