First, even with your regular setup with CO2 turned on during the day and off at night, you are fluctuating between pH a lot during day and night, this is not good.
i guess you need more carbonate buffer to cancel CO2's impact on pH during the day.
Due to low buffer, CO2 being acidic in nature, drops your pH during the day.
At night with no CO2, pH increases.
I would look into increasing the carbnate buffer enough so that CO2 does not impact pH during the day. Target atleast 4dKH.
as suggested above add NaHCO3 (Baking Soda) to increase the buffer to stabilize your pH.
As for why pH dropped overnight, you are running too low of a buffer, any minor change can cause the pH to drop drastically...it would be hard to pinpoint what caused it to drop.
Last edited by rrattani; 10-27-2014 at 07:48 PM.