I tihnk water quality is the prob. As to have so many diseasees in a short period of time either means that they have a very very weak ammune system, you bought a sick fish, or it's your water( thats in my oppinion). Make sure you check the water and post the stats.
Sorry, didnt see you already posted the stats. Okay, it's water quality. You need to do ANOTHER water change , and then pos the stats. In experience, the safe zone for nitrate is about 40 ppm. Yours is double that. Any amount of ammonia is Highly Dangerous. Your danio is probably suffereing form an ammonia burn.