ah crap, forgot to take a picture of the container. it’s one of these. The one i have is handwriting saying 1/2 tsp every 20 gallons
Oh - you are going to have difficulty holding the fish in that small of a container. Most antibiotics require 5 days of treatment. Also, that medication is going to be really difficult to "downsize" for that small of a water volume. If the container holds two gallons of water, then you would dose it with about 0.3 grams of medication - tough to measure! One trick would be to dissolve the 1/2 tsp of medication in 20 ounces of water and then use one ounce of that per gallon to be treated.
Jay