Calcium high pH low

[nate_newton]

Hey Beakerbob, I know you're one that'll know the solution and maybe a cause to this. I have a 24g cube, using Seachem Reef Salt. I do 20% water changes weekly, because of this I haven't been test my water in 2010 because the tank has been doing great. The tank is made up of macro algae, monti's and birdnest. I've been testing my water for a few weeks now. Here's today's, pH 7.8, dKH 10, Ca 500, Mg 1350, and water at 1.026, temp 79-81. The only thing I'm adding is carbonate, calcium isn't affected at all and stays at 500. I'd like to get my pH in the 8.1-8.3 range. Corals are growing great. Any ideas why my Ca stays up so high?

[BeakerBob]

Nate,
The relationship between calcium, carbonate and pH in the tank show that if you affect one of the parameters, you may affect the others.

Let's take your questions one at a time.

Calcium: normal salt water may have a high calcium concentration. It is in much higher concentration that the carbonates and is excessive in the relationship identified above. It is also easier to get calcium in the water than it is carbonates. All hard corals and especially SPS need calcium to build their skeletons. If you don't have a lot of SPS corals actively growing, your calcium levels will not go down much.

Carbonates (alkalinity): Not found in excess in tank water. Carbonates are required to complex with calcium to form the skeletons of CaCO3. Typically, carbonates are the limiting factor in the relationship identified above. This means that if ALL the carbonates were to suddenly bind to calcium in the tank and precipitate out, calcium would still be available in the water.

pH: Adding carbonates (baking soda) to the tank liberates CO2 that causes the pH to go slightly acidic. If this is a problem, put 2 1/4 cups of baking soda on a sheet pan and bake it in the oven for 1 hour @ 300F. This drives off the CO2 and any water that is present. When finished, dissolve the baking soda (now soda wash) in one gallon of warmed water and use. This solution will allow your pH to remain higher when you add the carbonates. Other reasons for a lower pH include a tight house that does not allow for CO2 to dissipate.

[jimsflies]

pH can also vary throughout the day depending on the photosynthetic activity (consumes CO2...raises pH) or respiration (produces CO2...decreases pH). If you monitor pH continously you will typically notice pH to be lowest right when the lights come on in the morning because during the night photosynthesis is typically 0 and CO2 builds up overnight due to fish respiration, etc. This is why many reefers will run a reverse photoperiod on their refugium in an attempt to balance the CO2 levels between day and night.

What method are you using to measure pH? Are you using a probe or strips? How often are you calibrating?