What can drop PH???

[barracuda]

Hello!!!
I'm used to measure PH on regular basis. It was always about 8.3-8.4 days and nights. Today i measured PH as always and found 8.2 during the day. I didn'd pay attention, 'cause it is actually good level. 3 hours after the light gone off i remeasured PH again and it was 7.8 .
I measured ALK, it was above 3.6 meq/l(this is the highest level of my test kit). What can cause this sudden PH drop? <img src="graemlins//confused.gif" border="0" alt="[confused]" />

 

[barracuda]

It's not me who multiplied the question....
Sorry!!! <img src="graemlins//uhuh.gif" border="0" alt="[U-Huh]" />

 

[ed r]

Did you change anything? Have you used any additives? Did you do a water change? Has any creature died or disappeared? Have you rechecked the test? Sometimes an error in the reading can occur. Some test kits are better than others. The 8.2 reading would not have been a concern, had it not been followed by the 7.8. Did this occur in a 110g tank? I would think it would take a lot of something to make that sudden a change? What are the ammonia, nitrite, and nitrate readings? Have they changed at all? These are just some things to consider to narrow down the possibilites. Good luck.