It sounds like columnaris, a deadly flesh eating bacteria.
Treatment for me would be oxytetracycline flake if they are all still mostly healthy, or you should use maybe Maracyn 1/2 combo
Not all bacteria is the same, and not all antibiotics work the same.
Not all will work in hard alkaline PH calcium water, most work best in soft or neutral water
Many people will say use kanamycin + Furan green, but furan is a harsh cancer causing chemical that i and not gonna reccomend to anyone anymore. Dioxin is what they put in defoliant, furans are among the harshes chemicals to use. Go the safe route and try the flakes or maracyn combo.
Remember, if you dont feed, you dont need to have cycled water. If you do feed, you can add bacteria to the tank by simply rinsing off a used spoonge or filter pad and pouring that bacteria into the new or hospital tank. Boom instant cycle (mostly)