Ph Fluctuation

[stanleo]

55gl saltwater tank. 3 days ago the pH was 8.3. Did my weekly WC of 20gl today. Tested pH after WC and pH was 7.8. I mixed 2gl of saltwater to test the pH of newly mixed water and the pH of the new mix was 8.3.
 
Why did this happen? I left the 20gl of newly mixed water sit for almost 24 hours before I did the water change. It's frustrating because this has been stable for almost a month now. dKH is 9.

 

[sputnick]

I've heard about bad salt mixes causing problems like this, did you use the same salt from the same batch? Otherwise it could be you water 

 

[Donya]

Was the newly added water aerated? If not, that can cause some pH weirdness that will sort itself out over time. Also, did you test the pH of the tank literally right before you did the WC that resulted in 7.8? If not, then my first guess would be something like a death in the tank or decaying uneaten food causing a pH dip.