Do you think that it is smart to admit that you sell antibiotics over the counter without labeling them consistent with the contents? While the FDA nor USDA do not currently require a prescription to sell for ornamental fish (aquatic life not for food), the do require that you label the type, amount and give approved directions for use to follow. Mostly... there are some antibiotics that are illegal to sell without a Rx for any purpose, but these are more specialized.
I am sure that it will be less than 3-5 posts before somebody claims that you don't have to label this stuff... but know that I heard this from the horses mouth while discussing another case. This is why you see the big companies like API, Fritz, etc. let you know what is in their products.
With the uptick in people circumventing doctors and prescriptions by self medicating with fish antibiotics, discussion is happening to potentially close this loophole of not having an Rx for ornamental fish. Apparently, the only thing allowing hesitation is the quality of the fish meds which are composition-wise identical to human meds, at the moment.
Do yourself a favor and label it correctly. People will still buy it since it is not easy to get in small quantities, honesty will push people to your brand over others and if a storm comes with fines or punishment from the USDA, then they can move to other people. UWC was cautioned to label their product correctly for years - please don't be dumb like they were.