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pH, pH scale -- ((chemistry) p(otential of) H(ydrogen); the logarithm of the reciprocal of hydrogen-ion concentration in gram atoms per liter;
provides a measure on a scale from 0 to 14 of the acidity or alkalinity of a solution (where 7 is neutral and greater than 7 is basic and less than 7 is acidic
Acidic - the pH is between zero and 7.0
Neutral - the pH is 7.0
Basic (also called alkaline) - the pH is between 7.0 and 14.
EDITED- thanks for pointing that out melissa
several factors determine your ph. i would suggest a ph monitor. and alkalinity tests i like fastest's alk test kit ,you want an alk of 3.5-4.5.
alkalinity is the same thing as KH (carbonate hardness) is the indicator of pH stability. again proper alk will help your ph to be more stable
i also like b-ionic's two part buffer system...one is for alk and the other for calcium{salifert test kits are reccomended for calcium}
Half a teaspoon of baking soda per 25 Gallons will raise the kH by about 1 dH (17.8 ppm). you can also make limewater .
i think you should goto
Reef Central Online Community and read some..very nice site similar to tonmo- tons of info.another usefull site is
Wetwebmedia, Aquarium, Pond, Marine and Freshwater Fish, reef tanks, and Aquatics Information
i hope this helps some. do you already have an octo?
EDIT----ps.. calcium has no major role in ph.