Short answer....gram negative and gram positive bacteria just refer to the type of bacteria by gram stains and the medication that is effective in the treatment-lots of different bacteria/pathogens- both good and bad that are in the water, on the fish, inside the fish...not every antibiotic will be effective against every bacteria/pathogen-just as antibiotics can't tell the difference between good and bad bacteria/pathogens- so antibiotics kill all of them-good and bad that the antibiotic is designed to kill-be it gram neg or gram pos bacteria-this often includes the good bacteria that is responsible for the nitrogen cycle, the antibodies that keep the fish and water healthy...etc.....
Gram negative bacteria is the most common bacteria that infect freshwater fish
Over use and miss uses of antibiotic can cause more harm than good-pathogen build/develop resistant-the antibiotic kill the good pathogens....etc....and soon resistant is developed and the antibiotic are no longer effective in the long term and the fish is even more compromised because nothing will work
Using antibiotic "just because" often causes more harm than good for both the fish and the environment
Don't use antibiotic unless you have a firm diagnosis and know what you are treating-use the antibiotic for the right reason, duration and dosage
Antibiotics have a short shelf life and its not recommended to keep them on hand...waste of money and time-if they are not kept properly they can go bad and/or be ineffective or weakened to the point that it can make the bad pathogen stronger/resistant and by killing the good pathogens that kill the bad pathogens
Best treatment is Prevention
Most common cause of health problems is environment related, Poor water quality/husbandry
Best medication is-clean dechlorinated water and water changes
Freshwater fish thrive with fresh water
To be a good keeper of fish you must first be a good keeper of water....