First of all, your nitrates are normal. Your ammonia levels are as low as an API kit or any kit can actual measure due to the human interaction of getting a sample. It is not that usual to have a species specific disease and since your danios are only species affected that would be my guess. I would not try to save the ones left by medicating the whole tank. Let them die off and try some time later to reintroduce. Just my take on this. Could be wrong or right.