Balance of Alkalinity and Calcium

[Frogger]

I have been working hard to maintain a consistent alkalinity in my tank. I have also decided to run my tank at a alkalinity in the range of 7.5 to 8dkh to reduce the problems associated with higher alkalinity levels.

About a month ago I changed from dosing B-ionic to using a bulk brand of calcium chloride and soda ash because my magnesium was getting high and I wanted to stop adding magnesium for a while to see if I can reduce it.

I monitor my alkalinity almost every day and adjust the amount accordingly to maintain a stable level.
I only check my calcium level once every couple weeks. I always add equal amounts of the 2 part.

My calcium has dropped from 420ish to now around 360 in the past month. Last night I checked the instructions on the 2 part and it says add equal amounts to maintain a level of 420 calcium and 9dkh.

What I need to know is this the ratio that adding equal amounts of this 2 part will maintain. Which means that if my alkalinity is now 7.5 instead of 9 (83%) then my calcium will be 350 instead of 420?

Seems pretty low. Should I add extra calcium to bring the values back up closer to 400ppm?

If the calcium and CO3 being used is at this ratio should I be running my tank at 350ppm calcium? No one does. If I bring my calcium up to 400ppm am I creating an imbalance in my aquarium?

Let me know if this makes sense or am I completely off base. I have not seen this discussed before.

 

[Randy Holmes-Farley Verified Community Expert]

No, you should not run the tank at 350. I recommend 400-550 ppm calcium, regardless of the alkalinity target.

Use just the calcium part to boost it over a few days.

It may have dropped for many reasons, most commonly water changes with a low calcium mix like IO. But if that keeps happening, dosing more of the calcium part is fine.

 

[Frogger]

Thanks Randy for the input. I am just trying to get my head around this.

If the Ca and CO3 are being used at a balanced rate how can we maintain the same level of Ca when there is such a large variance in the KH we are keeping our tanks? Wouldn't Calcium become the limiting factor when the KH is kept high at lets say 11 and wouldn't the reverse be true for a lower KH tank?

 

[Randy Holmes-Farley Verified Community Expert]

Seawater has, relatively, a huge amount of calcium and not nearly as much bicarbonate/carbonate/alkalinity. Whether that is part of the explanation or not, corals seem to not be limited in their ability to take up calcium (when you have at least natural levels), relative to alkalinity, and alkalinity (and pH) have been shown to be the limiting factors to calcification.