One time the goldfish in tank #2 developed white spots on his skin, and he became lethargic, hiding in the cave of the artificial rock. We diagnosed the disease to be Ich. According to this opinion, we shouldn't use chemicals more toxic than salt to treat Ich, so perhaps you shouldn't use antibiotics. Apparently, Ich is supposed to respond quickly to salt -- three to seven days. So, we decided to treat the Ich with salt water treatments, and warmed up the water a little (there is an aquarium water heater in the bottom drawer, under tank #1).
On this web page, they say one teaspoon per gallon for three treatments, and in parentheses they say "3 tsp per gallon", indicating that after the three treatments you have an equivalent of three teaspoons of gallon. Consider leaving the water salty until the fish seems cured, up to the maximum of seven days. Then do a 50% water change weekly for the next four or five weeks; each 50% change will reduce the salt content by half, so after the fifth change it should be down by 1/32.
We were lucky, and Erin's goldfish recovered. Helene once had goldfish that weren't so lucky.
Hope this never happens under your watch! If the unfortunate happens and the fish die, do NOT wash the tank with soap. I read somewhere that if you do this, it will be very difficult to get all of the soap out of the tank, rendering the tank permanently poisonous to fish. If you Google the topic, you might see suggestions to clean the tank with bleach or a strong saline solution. I'd go with the saline.