Re: KH of 2 too low for non CO2 tank?
Leave it alone.
Non CO2 means no water changes and with Discus, while possible, most over feed them, put too many into the tank.
I'd not have more than a pair, maybe 4 tops and no heavy feeding.
No KNO3 either, just PO4 and EQ and tad of traces once a week.
I'd use lots and lots of plants and put some shade plants on the surface and pack the darker areas with more plants like Java fern, Anubias etc.
I had 5 fish well fed in a 90 gal CO2/high light, I migth have been able to add a couple more...
Regards,
Tom Barr