The oceans change, but very slowly. Your calcium/alkalinity/magnesium numbers will fluctuate depending on the stoney coral loads you are supporting. A minimal load of SPS
can be accommodated by occasional water exchanges. But higher loads require testing and dosing. Just try to not get a see-saw effect going. 9dkh is fine, 400+ calcium is also fine. Check Mg
sometime to insure it's in balance as well. My tank for example gets kalkwasser to keep the average hardness up, but I fine tune with dosing both calcium as well as sodium carbonate to keep hardness and alkalinity in balance. But I have approximately 100 pounds of stoney coral colonies. That 100 pounds of carbonate rock was created out of the ingredients in my saltwater.
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