15 years of experience and I'm stumped

[Psirex]

For the last three days my PH meter reads 7.3 at lights on. It has never read less than 7.8, even in winter with windows closed. Within an hour of lights on, it recovers to the typical PH cycle eventually reaching 8.3.

I have checked calibration, ALK, Calc and all params are fine. I dose BRS 2 part via LM 3. All coral 'canaries' and fish are acting normally.

About a week ago my large Derasa had a spawning event which my two skimmers handled without issue in my 210g mixed reef with sump. Yesterday my skimmers went crazy, were back to normal by this morning but the PH still read 7.3.

I have attached an old pic for reference. Suffice it to say the clam is another 50% larger so it was a good sized 'event'.

The closest I can figure is a bacteria bloom? But then why such a strong daily PH recovery?

Recent pic of the clam!!!!?!?!?!?!?!

 

[TampaSnooker]

It is a possibility but the acidity would not simply be present at night.

Sorry - I overlooked the day/night swing and just saw it coincide with the spawn. How about household CO2? Are windows open by day and shut by night?

Edit: I just reread the thread. You've got plenty of corals whose photosynthetic activity might have corrected the pH on a daily basis. The fact that the issue cleared up within 4-5 days is consistant with the time required to clear ammonia and degrading proteins from a single event (spawning, overfeeding, fish loss). To me, it seems that your figures were too consistant and self correcting to blame it on the probe. It is perfectly logical to search for corroborating events.

 

[reefkeeper2]

Increase in bacterial populations due to the spawning event. With the lights on the tank could handle the extra CO2 they produced. When the lights went out the pH crashed. Now that time has passed their numbers have returned to normal and so has your pH.

 

[buckster]

+1 Increase in bacterial populations due to the spawning event.
When in doubt, change some water out. Since the skimmers went a little crazy I would do a partial water change just to be on the safe side.