"In chemistry, pH (potential of hydrogen) is a numeric scale used to specify the acidity or basicity of an aqueous solution."---wiki. Ie, is your water acidic like lemon juice---or alkaline like a desert pool?
On that scale, battery acid (ph of 1) and drain cleaner (ph of 12)---both extremes can burn.
Your tank should be between 7.9 and 9 for the comfort of fishy skin and gills.

BUT--- ph in a marine tank wanders all over the map during a given day. Pretty...