Hi Fig. That's a very broad question, and a forum post can't do it justice.
Don't worry about GH for now.
Nitrites are really bad, they should be zero, but if you've got just one Danio and a bunch of plants, your Nitrites are probably zero.
Here's one suggestion: measure your tap water's pH and KH and then your tank's pH and KH, and you will know how well your Hagen thingy is working. Good luck!