PH is always going to be lower in the morning than in the evening. This is normal. This is why some people will run the lighting on their refugiums on a schedule that is opposite the main tank in order to balance the PH. A drop in PH during the evening is part of the respiration process. Your PH should swing less than .2 during a 24 hour period otherwise your animals can become stressed. 8.15 is a little on the low side but not by much. 8.2-8.3 is recommended.
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