My water comes out of the tap @ 7.2 with virtually zero GH and KH so I let it simply sit and watch it drop to around 6 then I buffer it to 6.8 3 GH 3 KH and 28c for all WC's. For UV's the only thing important is flow rate per watt. 99% of people who use UV's do not understand that the rating for actual sterilization is about 10% of that quoted by UV manufacturers as they are quoting rates for use in ponds for anti algae treatment and not sterilization use in a tank. I use @ 55w DeBary UV mated to a Eheim 2217 (which has no bio media only pads and chemicals) which in real life gives me about 450 l/ph of flowrate. I use the 10 watts per 100 liters per hour of flow as a minimum for correct UV sterilization so I'm slightly over the flow minimum.
I recommend DeBary , Oase and for a cheaper brand Pondmaster UV units and anything under 36 watts is useless unless your planing on very low flowrates.
In my experience Nuggets and all Bary species are not difficult fish at all to keep and grow (breeding is another matter) and their reputation for being difficult mainly comes from people who are ignorant in their needs. Miss a WC ...even one with these fish or don't use UV's or chemical filters and sure ....they don't grow and die easily but do it right and they like Tigrinuses (another fish with a false reputation for being touchy) or most any white water fish they are kept easily.