DO NOT USE WINDEX (or any other standard cleaners). This is obvious to some people, but it's super important.
There's a line of products called Doctors Foster and Smith that makes great products for cleaning aquariums.
Also, do some research on "cycling" your tank, and you can avoid your issue with the danios. Also, since you didn't mention it, fish need to be acclimated (another process to research), and your water needs to be treated for chlorine and chloramides. For the latter, I recommend Seachem (company) Prime (product).