Disease in community tank, need help with meds

[IHOctopus]

 

I have a ~150 gal tank that I maintain at work. Inhabitants are hatchets, bloodfin tetras, denison barbs, SAEs, yoyo loaches, and harlequin rasboras.  77C (no heater), nitrate=50, nitrite=0, hardness over 300, buffer=40, pH 7.  Test strip pic attached. I change ~35% water every 2 weeks. No new fish added this year.

A few weeks ago I noticed one hatchet had a swollen lower jaw, but it was still eating and acting fine. Last week the scales on the face were gone, and the jaw was red and swollen. (Did not look like fungus.) I placed him in a quarantine tank and treated with salt ("level two" dose) and one dose of erythromycin. This is a picture of him after 1 week of treatment. Jaw is no longer red, but still swollen.

Today I noticed the bloodfin tetras are now showing similar symptoms. Lower jaws are swollen so the mouth doesnt close. Those tetras also have redness around the gills. I tried catching them out, but they were too fast and the tank too big. Pic of them hiding in the left corner.

I did a 50% water change and added erythromycin. I didn't want to add salt because of the plants.

Anyone know what this is? And should I be combining it with another med? When you guys do erythromycin, do you dose daily like the box instructs?

 

[Colu]

With symptoms your describing I would treat for  bacterial infection it could possibly be  aeromonas that's a gram negative bacteria what I would do is treat with a combination of maracyn2 in food feeding a small amount twice a day for 7 days  active ingredient minocycline also has anti-inflammatory properties and dose the tank with metroplex @IHOctopus