Carbonate Hardness

I know it isn't always very comforting to get links thrown at you, but give these a read to begin with.

http://faq.thekrib.com/Annex/oldfaq-hardness.txt

http://www.thetropicaltank.co.uk/hardness.htm

Lots of info out there. pH is a measure of acidity or baseness of a solution--how many hydrogen ions are drifting around and how they are charged. KH has to do with the amount of carbonate buffer in the water. Higher amounts 'buffer' the water, meaning the hydrogen ions tend to remain at a steady level and state. For saltwater, the numbers are a bit different than for FW tanks, since trace minerals and salts buffer water as well. In SW tanks, alkalinity, which includes more than just carbonates, is used more often than just KH.

Here is a decent explanation: http://saltaquarium.about.com/gi/dynamic/offsite.htm?site=http://www.amdareef.com/ho_chem1.htm

oops, sorry hunk. I didn't see you had posted in the marine forum!