Bought a Nitrate vessel to make my Discus Happy......im confused?

Hello Everyone,

Sorry if this turns into a long question but I will try to keep it brief, I am currently running 4 tanks, 1 is a Rio 180 running off a Rio 180 sump with 16 x 5" Altum Flora's in it, the 2nd tank is a 18 x18 breeding cube with 14 x 3" Albino platinums in, the 3rd is also a 18 x 18 with 8 x 4' Albino Tiffany Yellows in and the last is a small 20 gallon with 10 x 2" Red Covers in, I do 80% WC on all of the tanks daily, the platinums and the red covers sometimes twice daily, I am running my tap water through 3 large 20" HMA filters (60%) and a RO filter (40%) 60/40 mix........today I bought a Nitrate resin vessel to put in line after my HMA filter as instructed by the seller, the thing that is confusing me is that he said to slowly over the course of a few weeks stop using the RO and my water will be harder as at the moment my water is quite soft, but I thought it better to have soft water for discus? all the water I make is stored and heated overnight before using, I have my WC's off to a good routine but this nitrate resin vessel is confusing me as it seems the water will be much better quality running it through it but it will be harder, how can this be better for discus?

Thanks
Sorry again for the long question. :-)

P.S These discus aren't staying in these tanks, they will be going in bigger tanks, I am in the middle of building my fish room, hence the big filters and the resin vessel.