I completely agree that this could be a good addition to the forums. I'd have 1 section called water chemistry with 4 subsections. Marine, brackish, coldwater and tropical. Some of these sections might not get used a lot at all (marine & brackish) and might be better off in the for sections already on the forum.
However, tropical and coldwater would make a very good section for starters to check and compare their readings to those of other tanks. It will also be easier for them to find a thread in which someone has the same problem with their water than it will be to find the exact problem in a pinned topic. A lot of people do not have the patience to read through a pinned topic (if they can even find it) and if they do, it's often hard to understand because it isn't quite written as a one person to another problem guide.