I definitely wouldn't use all RO
. 50% tap should be fine and a lot more stable. I wouldn't trust Brightwell's stuff, especially not as the only thing used to reconstitute 100% RO
We used half tap and half RO
at the shop and the discus were breeding in the sales tanks.
Discus are not the super sensitive fish many make them out to be. They need high quality water, like any fish. But they will breed with alkaline pH.