A healthy growth of caulerpa with good lighting 24/7 in a 20 gallon fuge for a 110 tank can take your pH to over 9.0. It won't come down so long as the photoperiod is 24 hours. It will only fluctuate because of the added algae contribution from the main tank during the daytime. Once you're done calibrating, testing and testing some more, if you come up with no solution, start cutting time off the fuge photoperiod in the late afternoon. An hour at first, then extend by 10 minutes every other day till you get to at least 6 hours of dark during the daytime. That will help your pH out and do so gradually within the daily fuctuations that are already occurring. You can check the variation in the fuge by finding a part that circulates with the main tank water the least, near the surface and in a dense patch of caulerpa and test the pH to see if its a bit higher than your tank. If the meter is sensitive enough and you pick a good spot, you'll see it. Don't start on the photoperiod until you're done testing and calibrating as mentioned above in case it is just the pH probe being a little off.
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