One of the fastest moving bacterias is the flexobacteria, two type, the fast moving one and the slow moving one....the fast moving type can hit and kill the fish within 24h, the slower one the fish can recover with treatment.
This bacteria is ever present in our tanks just waiting for the right conditions to infect our fish, sudden swing in temp, poor water quality, fish stress are the most common reasons.
It thrives in warm water, so get the water temp lower 72-75F, move to small QT and making 100% water changes daily and adding 1tsp/g of aquarium salt up to 3tsp/gal can help, feeding the antibacterial fish food soaked in fresh garlic juice, adding an airstone to increase oxygen to water.
Flexobacteria will look layered, columns, sometimes fuzzy look to it as well, it can start in/on the mouth and spread very fast to other parts of the body and look like the flesh is being shed, holes and creators can appear, it can look like a saddle on their back fish will be lethargic and hide and usually stop eating,
Use of a broad spectrum antibiotic can help if it is the slow moving type.