I just picked up one of these Red Sea drop checkers locally -- it's certainly a workable design, but I can't believe they can instruct people to use the tank water. I started out following those instructions before doing some more reading online (and discovering I overpaid, among other things). It's a good thing too, because I have hard water with about 15 degrees KH. The fish would have been long dead before it turned green.
I was afraid that mixing my own 4dKH solution was going to be a pain, considering how the recipes are numerous and lengthy. As it happens, I tried starting with the water from a cheap (eBay purchased) RO water filter system, which produced water with a tested 4 (maybe 4.5) dKH. Time saver!
I tossed a bit of the RO water into it instead, and after a couple of hours the solution has settled into a healthy green with an ever-so-slight hint of blue. That's close enough for me, since I'm using DIY CO2. (Two 2-litre bottles w/ bubble counter, feeding Fluval FX5 canister. About 120 bubbles/minute. Some surface agitation from a AQ 70/300 HOB in a 75 Gallon tank)