I read in another post of yours that your tap water has a Nitrate level of 40mg/l. I would first double check your Nitrate readings with another test kit or take a sample to your lfs to test.

If that level is confirmed, why don't you invest in an RO unit instead of this? That'll give you a nice clean base water (with 0 of everything) to start with and then you can either mix it with tap water or reconsititute it yourself with CaCl2, MgSO4 to get the GH up and baking soda (NaCO3) to get the KH up...

Voila, no nitrates! Then it would just be a question of water changes to keep the Nitrates down.

I'm also skeptical that any fish absolutely requires a Nitrate level of below 20 in order to thrive. I keep my planted Discus tank at around 25 to 30 with no problems. Tangs are a lot tougher than discus...