Hey Marc,
The way I understand it goes like this.
Calcium Ca+ will be used up as hard corals and coralline algae use it for skeleton formation.
CO3- carbonates will also be used up by the same living things.
If you don't have a lot of coralline or hard corals - once you get the calcium level established - it should remain fairly stable.
You may still need to add some form of calcium now and then - but for the most part it will line out.
Carbonates on the other hand ( alkalinity ) is the "buffer" components in the tankwater.
As fish and other creatures live, produce wastes and stuff - they add to the total acids that are in the tank. This stuff drives the pH down.
As CO2 gas is brought into the tank at air/water interface - it too can and will affect the alkalinity.
CO2 and water will produce carbonic acid H2CO3 - which can drive the ph down too.
But CO2 can also lead to formation of bicarbonates and ultimately carbonate. This can increase the buffering of the tank. But in doing so - it realeases H+ ions ( again causing the tankwater pH to drop ).
So ......... it's pretty common in tank with very few calcium demanding critters - for the calcium to be more stable.
On the other hand - as the pH is affected by fish, CO2 gas, formation of acids, changes in the carbonate/bicarbonate buffering - the alkalinity WILL drop for sure.
The rate at which it will drop will be different for each tank.
Some will drop quicker - some will drop slower.
The buffer capacity ( alkalinity meq/L or dKH ) gets used up as it "neutralizes" acids in the tank. Alkalinity will drop in a tank with fish.
So we gotta add something to keep it up there.
All depends on what's in the tank, how much CO2 is in the tank room, how much CO2 is in the water, temperature, calciim levels, and so on.
EVERYTHING - calcium, alkalinity and pH levels are linked together by what's happening with the CO2 gas, and each other.
It's a trip for sure.
