Big patch of white discolouration at dorsal fin base

[Ash1176]

I’m wondering what might be wrong with my German blue ram.
Tank is about a month and half old, fishless cycle, fish added this weekend.
Ph 6.8, ammonia 0, nitrite 0, nitrate 20ppm
Temp: 80, 20 gallon high
Just noticed this tonight, his dorsal fin is really folded or gone, and there’s a big white patch on both sides. Fish looked perfectly fine this morning. Is it fin rot ? Looks like it’s kind of dry, there is also a bit of white on the left fin.
Lost 2 cardinal tetras over the last few days, also got them this weekend.
Any help would be appreciated

 

[kallililly1973]

Welcome here. Not an illness pro but looks like columnaris and appears all your fish may have been infected by it seeing your losing fish.

 

[Ash1176]

Thanks for the fast reply, yep looking at the description “The disease in the past has been called Saddleback Disease because often the afflicted fish will show a pale saddle like area across their dorsal finonto their back” that pretty much fits the bill.
will try and find API Furan-2 or Tetra Fungus Guard or Jungle Fungus Clear tabs tomorrow

 

[Ash1176]

sadly the ram passed overnight...
I’m on the fence about treating the rest of the fish in the tank, they don’t show any signs of Columnis as of yet, but I understand it can spread, Would exposing the healthy fish to the meds outweigh waiting to see if any other fish get the disease ?