4DKH water can be bought from one of our sponsors aqua essentials as well as others. I made my own up using the method in the other post that is linked. It will be more accurate than some home made stuff but like that post suggests it doesn't really have to be that accurate although it will help. Testing by using double the amount of water than your test kit recommends will give you an accuracy of within half a degree of KH or you could keep it going and get further accuracy still by adding more water to the test. That all depends on the accuracy of the kh test kit, electronic ones are better and quite cheap these days.
That said, the best test kits are the plants themselves. They will let you know if you have enough and the fish will let you know if you have too much by gasping when the concentration gets too much. The most important bit is keeping the levels stable day in day out and ensuring that there is enough when the lights first switch on. You can in this way have the colour which is right for you, for instance if you realise that your DC even though is lime green isn't enough co2 because your plants are showing co2 deficiency then run it on yellow as long as the fish take no harm. Just don't take for granted what the DC is saying colour wise especially if you do make you own batch and it may be slightly out.
Bromo blue is generally accepted as being the best indicator fluid because it works in the range we are wanting to work at between 6.2-7.6, I can't vouch for any others but I know Nutrafin low range ph kit has bromo blue as it's only ingredient.
