A 55 Gallon tank is far too small for 3-4 large growing Cichlids. You would need to perform 30-40% water changes 2-3 times a week just to keep your bioload on an average scale. Also, an Oscar will certainly outgrow a 55. Realistically, one or two Parrot fish and a Severum would MAX out a 55.
I would try to treat these fish and then re-home a few so that you are left with just two and definitely move the Oscar. You could then do one weekly water change and be back on the safe side. Even with great filtration, a 55 just doesn't have the space for your stock when fully grown. They look pretty big in the pics you have provided.
As for this potential (likely) disease, check your water conditions and continue to monitor the situation over the next few days. I have seen this problem before in some LFS, but personally, I am not sure what the diagnosis is. Wish I could be of more help in this regard.
