Oxytetracycline (TerraVet) is not the ideal medication for Myco and most infectious respiratory diseases. It's just the most available one. The ideal medication is Denagard 12.5%, which you'll have to order online. It's probably ten times as effective. Dosage is about 1.5 teaspoons per gallon of water for 5 days.
Please remember that any medication for Denagard is only a treatment and not a cure for Mycoplasma or any other respiratory disease. These diseases cannot be cured and unless you can confirm for a fact that Mycoplasma is not in your flock you should not sell or transfer any birds off your property. My recommendation would be to get birds blood tested if you ever do find a vet within reasonable distance; I know you can also draw blood yourself and mail it in to the labs, though I've never done it myself. A blood test is the most accurate way to confirm what you are dealing with.
