Very sudden Fin rot and flashing

I have a 55 gallon tank with 7 Tiger barbs, 2 CAE, 1 Rainbow shark, 1 Gourami. The last two days the Rainbow has been flashing but otherwise appears normal. Several of the barbs have what I think is fin rot, developing quickly two days ago it was small discoloration on the tail, yesterday it was large white patches and today they are missing chunks. They have been headstanding and their whole body looks faded, or grey...I am not sure how to describe it, almost like their slime coat is foggy. Some are also breathing hard with clamped fins and hiding, and none of them have their usual appetite. No other fish are obviously affected.


Tank Size: 55 gal
API test kit: (Tested weekly, no major fluxuations)
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 20 ppm
pH: 8
GH: 179 ppm
KH: 89.5 ppm
Freshwater that has been set up approximately 5 months
No new additions in 3 Months
76 degrees
No live plants
Emperor 400 filter
Weekly to Bi-weekly 20% water changes with RO/DI water treated with Seachem Equilibrium
Fish are a combination of flakes, algae wafters, shrimp pellets, and freeze dried blood worms 5 days a week.

I am using RO/Di because my tap water is very high in Nitrates to begin with, somewhere in the 60 ppm range. Yes I agree that it came on too fast to be fin rot, probably a bad way to describe it. It is the areas of the fins that had the white patches the day before that deteriorated.