I've used the stuff and Allivymar is right on the money... as usual.
If you test your water and your kh
is still high then don't waste your bottle of ph
down cuz it won't work... lol. I performed massive water changes with distilled water until I got my kh
down to around 5 (20% water change isn't going to do it). Then I used the ph
down to get the ph
to where I wanted it. It was trial and error for me on how much. As such I'd just follow the directions and keep testing the water. You'll soon figure out how much effect the ph
down has for your particular water. Of course, I got lazy and bought a ph
regulator not so long ago which sets my ph
at 7.0 automatically for me!
If you aren't willing to use natural means to regulate your ph
then I'd go for a ph
regulator like Bullseye 7.0. Just keep your kh
down and include the regulator in your water changes (so it doesn't get diluted) and you'll be fine.