Alk and Ca Dosing way out of sync

[StevePhx]

I’m really struggling to figure out what my alk and calcium usage is so far off?

I’m dosing 150 mL of alk and 90 mL of calcium. BRS soda ash and sodium bicarbonate. Alk is at 9.0 and calcium at 490.

They are both being dosed by an Apex DOS. Alk is done 12am to 8am. Calcium is done 9am to 9pm. Both are dosed into the return section of sump. High flow and no precipitation in sump.

Tank is 130g with 50 sps colonies from 1” frags to softball sized colonies.

Testing both with Hanna and double checking calcium with Red Sea.

Bi-weekly 20% water changes with TropMarin Pro.

What am I missing?

 

[Travis Stewart]

Have you tried checking your calcium levels of your fresh salt mix?

 

[StevePhx]

Have you tried checking your calcium levels of your fresh salt mix?

Trop Marin Pro tests at 410 ppm

 

[Dkeller_nc]

You shouldn't necessarily expect your calcium and alkalinity consumption to be in a 1:1 molar ratio, because carbonate is part of the nitrogen cycle. Specifically, some carbonate gets "consumed" in the oxidation of organic molecules to nitrate. You get this alkalinity back if anaerobic bacteria in your tank are reducing nitrate to nitrogen gas, but this might not be occurring at a fast enough rate to balance the equation.

There's a couple of other possibilities. The first one's easy, and that's examining whether your DOS is correctly calibrated. Even if you did it when you installed it, the calibration needs to be repeated every so often. Another possibility is that your alkalinity and calcium supplements aren't really balanced. This could occur if the particular recipe you're using wasn't correctly calculated, the materials you're using are different than the assumptions made in the actual recipe (for example, the recipe assumes anhydrous calcium chloride, and you're using calcium chloride dihydrate), or the accuracy of your measuring devices are suspect. I have a chemistry background and thus have laboratory equipment at home, so I make my solutions with volumetric flasks and by weight. But most published recipes assume you're using volumetric measurements of the salts (such as "a cup of baking soda") and an approximate measure of the dilution volume, such as "fill the gallon jug". These measurements aren't terribly accurate, so it's possible that your solutions aren't really balanced.

But the bottom line is that dosing should be done by testing, not by volumetric additions of the dosing solutions. If your calcium and alkalinity tests are accurate, and you're maintaining the parameters where you want them to be, and your corals are doing well, that's what matters.

OTOH, your calcium does seem a bit high for the particular alkalinity that you quote. Randy wrote an article specifically about correcting such situations in 2002. Rather unfortunately, the article hasn't been maintained all that well, so some of the graphs are missing. Nevertheless, the text will tell you what you need to know.