Your English is just fine ! I'm German and English is my second language, too ..

Sounds like you are doing all you can for your little snake. Shame on whoever took the poor baby to be sold while it was sick..

Unfortunately, a Upper Respiratory infection that keeps coming back despite treatment can be a symptom of a bigger problem. The only thing you can do is try to figure out what causes it, whether it is bacterial, viral or fungal. It seems your Vet is running tests, so that is great.

I would think if antibiotics helped him before, you could give him some now while you await the test results. But call the Vet first, and see what they would like you to do..

I hope its nothing viral or one of the diseases for which there is no cure. If it ends up being that way, you certainly did nothing wrong.

Maybe you just need to switch medications, sometimes a "bug" is resistant to some antibiotics and its hard to eradicate them. Baytril is not always the best solution, I'm almost certain that there are better options. I'm not 100% what its called or used for, but there is a antibiotic out there that has to be kept frozen until you administer it (right after it becomes liquid again). I hear it works wonderfully.

Make sure you run a culture to know exactly what you are dealing with and then discuss other medications and whether they may be a better option.

Good luck