That's your main issue. That's too many fish for that amount of water. The bell's Angels alone need more than that amount of water to keep them from getting sick. Medicating them in there is not going to do much good (imo
) so I would spread them out into multiple tanks to help them heal.
For what it's worth, a quarantine tank should be set up similar to a permanent tank for the fish to get used to their new conditions. If necessary, due to the fish getting sick, the fish should be removed to a hospital tank for medicating because many meds will effect the biological filter. A hospital tank should be a sterile, bare bottom tank that does not have any decor that can absorb medicines. In most cases, in a hospital tank, there also is no filtration. Just an airstone is all that's usually necessary because the med schedule also contains doing water changes so that a filter is not necessary.
I hope this helps.