While brook is most common, just want to verify what disease you are targeting.
If brook, typical treatment is a formalin solution is mixed with in a separate container with either fresh or saltwater. Start with a quick dip in the formalin at a higher concentration then performing treatment in a prolonged bath of formalin base at a lower concentration in a quarantine tank. The longer the fish are exposed to the formalin treatment the more effective it will be at eliminating this issue.
If a formalin solution is not available for immediate use, temporary relief can be achieved by giving the fish a FW bath or dip in water same temperature as display tank. Even though this treatment will not cure the disease, it can help to remove some of the parasites, as well as reduce the amount of mucus in the gills to assist with respiration problems.
Treatment is best done in a QT tank using either quick cure (more effective) or Ruby Rally Pro. Ruby takes a little longer and initial treatment generally takes 2-3 days to really start going to work.
With the advanced stage of this- I recommend immediate quarantine of all inhabitants and leaving display without fish for 4-6 weeks.
A quarantine system if you dont have one can be as simple as a starter tank kit from walmart which has most of the essentials