Terramycin soluble is notoriously ineffective. Don't waste your money.
Separate bird immediately is the good move.
Many simply immediately cull birds that show definite respiratory illness after quarantine. You need to come up with what seems right for you. I have never read an authoritative source that says respiratory diseases--period--are curable. In other words, the patient remains ill as long as it lives and can transmit the disease to others. This includes diseases such as Coryza, Laryngo. also.
The best treatments if you go the route of antibiotics, would be to use injection. This is the only way to be certain that the bird(s) you want treated are really getting treated. With water solubles, the bird must drink a certain amount of water each day to get the proper amount of med. That is very hard to provide, unless maybe it is hugely hot outside and your bird is driven to drink lots of water.
Gallimycin is approved for use in poultry, but as with Terramycin soluble or Tylan and other antibiotics, you must toss the eggs for a while before you can eat them; and even then, the withdrawal period is not known for sure, because studies re. withdrawal times have not been adequately done with chickens. The egg industry simply kills any sick birds, so they don't bother with treating..........
Unfortunately, Gallimycin is also powder. Tylan is injectable. But if you're gonna do antibiotics or just cull, is a policy you must think out for yourself. Treatment can easily cost more than a whole flock.........
Good luck
