whe ever there are things in your tank that photosynthesise then they take up carbon dioxide and produce oxyge. These can be simple life forms such as algae. This is the reason for higher carbon dioxide levels at night. Higher carbon dioxide levels, due to the carbon dioxide conversion to carbonic acid and its reactions with calcium cause decrease in pH, hense the difference between night and day carbon dioxide levels. Reefers with large night/day pH swings that are consistent usually install a refugium or at least grow macro algae in a lit sup with the sump/refugiumlight running in a cycle opposite of their display tank. This reverses or at least dimishs the day/night pH swing. However, your problem as previously stated is probably due to cycling. Your pH is on the high side of normal as said by Biffer. Make sure your water has aged four several days under circulation and bring the temperature up slowly to your display tank temperature. Mixed your new water up at least 12 hours ahead of its use, and preferably a full 24 hours or more ahead. A reefing expert or consultant would say that every reefer should always have on hand at any one time enough premixed, preheated water to do a complete water change on their reef tanks. Thats is the high end of safety and good husbandmanship. Premixed water has pretty much an indefinite storage life if kept circulating. Just do a water change and have your tap water checked, it would be preferable to use RO or RODI water from Walmart, PETCO, Culligan or a LFS, rather than tap water.
