Up and Down PH?

[saycheessee]

I have a 24g nano cube and also a 150g tank. I 'm puzzled on this one. In my 24g cube the Ph keeps normal at 8.3 to 8.4 after a water change.... but give it a week and it drops to 8.0 what makes this happen and why? I have never had this problem with my 150g (but it's also 1 year old and the nano cube is 6 weeks old.

[Randy Holmes-Farley]

Low pH is caused by either very low alkalinity, or excess carbon dioxide. Maybe it has poorer gas exchange, or the air around it has excess carbon dioxide. More fresh air and/or limewater are the best ways to raise pH. If the tank is still cycling such that nitrate is building up, that can decrease alkalinity and reduce pH.

This article has more:

Low pH: Causes and Cures
http://reefkeeping.com/issues/2004-09/rhf/index.htm