I wouldn't use antibiotics at all. It is better to requeen to make sure, that you have hygienic bees. The only ting you can be sure of, when using antibiotics as a prophylactic is, that you will build resistant strain og FB and get antibiotics in your wax.
Using hygienic bees and keeping the number of mites low in your hives can illiminate the problem. I know beekeepers who hasn't had neither AFB nor EFB in 20 years even though he is moving his bees around a lot.

Do NOT use terramycin disolved in water. It dissipates quickly, much sooner than the syrup is consumed. Worse, a boardman feeder exposes the mix to light which pretty much destroys the effect of terramycin quickly.

Requeening, while good advice in general, doesn't necessarily help with keeping AFB under control, as it is primarly spread by the workers, and can be brought in by bees robbing dead or dying hives within a few miles of your hives. Because tests have found latent foul brood spores in wax and propolis, and because it can lie latent for long periods of time, there is now talk about getting rid of old comb, thinking that the older it is, the more afb it can be harboring. (Inded I found old books bragging about 20 or 50 year old comb). Until recently, they were advising to cull all comb over 5 years old. By consistently replacing it with new (and presumably clean) foundation, it is assumed that there will be less latent AFB spores in the hive. Now, instead of 5 years, (that is, replacing 20% of your comb a year), it's now being talked about as being a three year cycle, and replacing 33% per year.