Can anyone identify this white stuff on my rainbowfish?

[Jescereal]

First of all, please quarantine your fish. I did a two week quarantine when I added new rummy nose tetras but it wasn’t enough it seems. As I added them I’ve been slowly losing fish at least once a day. Seems to have slowed down though. 
 

I lost a rainbow to whatever this disease is already. Can anyone with some experience help? I assume Columnaris so I’m dosing Maracyn. I’ve had these rainbows for a year now. No issues until I introduced the new fish. I’m only seeing this on the rainbows. 75g tank. 
 

• pH 6.8
• Nitrates 10
• Hardness 300ppm
• Nitrite 0
• Ammonia 0
• KH/Buffer 40ppm
• Water Temperature 77

[Colu]

It looks like a bacterial infection possible Columnaris. Columnaris  is a gram negative bacterial infection maracyn treats gram positive bacterial infections the best treatment is a combination of kanaplex and api furan2 he's the best way to treat Columnaris

If you can't get furan2 you can use AAP furacyn instead of furan2   if you can't get ether treat with AAP spectrogram it contains nitrofurazone and kanamycin the active ingredients in furan2 and kanaplex

Edited October 18, 2021 by Colu