I made my own 4KH water. I don't care that much about getting exact results - just so I can see if it's going up or down. The bba tells me if it's going TOO low....which consistently happens when I get to the darker green/blue green area. So I think my homemade preparation is fairly close. My plants tell me which color I want my drop checker...when it's greenish yellow, they grow like mad. That's all I care about. Good to change out your solution once in a while though, if in doubt. I haven't changed mine, though, in months, and it's still working just as when I first put it in.
I put carbon in my filter a few months back. I have no idea if it was coincidence or not, or what happened, but within days my hygrophila polysperma, which I haven't been able to kill for over a year of experimentation and messing around with co2, lights, ferts, etc., suddenly started developing strange new growth, it looks like parts of the leaves were actually chewed off (but they weren't). After a week or two, can't remember exactly, I put together that I added the carbon right around when this started. So I removed it. Since then, doing lots of ferts and upping co2, it's just now starting to come around and grow normally again. Coincidence, possibly, but I won't be attempting carbon again anytime soon. As a sidenote, my tap water has 0 GH and KH, and I have to add magnesium/calcium each water change for the plants. Not sure if there was something else there that was CLOSE to deficient (since my tap water is so devoid of anything), that the carbon maybe pushed over the edge into major deficiency? Anyway, I'm not risking it again. Also I should note that I decided to start using spring water instead of my tap water, it was really after I did the first water change with this that my hygro started coming around a bit more. Many people report having no problems with carbon, but many people also have fairly hard water that probably has a lot more stuff in it than mine does naturally.