I let the virus run it's course and use topical products like Vicks Vapour Rub or Vet RX to assist with breathing issues. Plenty of ventilation/fresh air/clean bedding and water etc. If you must use an antibiotic, then it is best just to treat the birds that are really struggling with the symptoms and are deteriorating or not improving, but a virus outbreak will typically take several weeks to settle down.How do you treat respiratory illness?
Do you know which respiratory disease you are dealing with?
Many will reoccur because the virus stays with the chicken for life and outbreaks are triggered by certain conditions or stress.
I'm quite surprised you were able to get an antibiotic like this without a prescription. I thought the new laws were supposed to prevent this, although I appreciate that there are always ways and means! I had wondered if the law might make it difficult enough for people to think twice about antibiotic use and only resort to it if absolutely necessary but I guess that when people have been so used to buying it and using it like paracetamol, it is a hard habit to break. It is probably a cultural thing. Here in the UK we need a prescription for them and always have, so they are very much a last resort and I think, whilst it makes it expensive when you really need them because it involves a vet, you at least get the appropriate one for the infection you are dealing with and don't use them if you don't need to. We all pay the price for antibiotic resistance and it can be a very high price for some people.