Dosing Questions

fishyman19 · Jan 11, 2018

I set up a 20 gal IM tank at work a couple weeks ago, that I'm looking to keep mainly SPS dominant with a few zoas and some lps in. I noticed at the beginning of this week that my dosing seemed a bit off and was curious if I could get some help. Not too smart in the numbers game with dosing. Here's where my numbers started at this week.

Mag-1260-want to keep it at 1400
Cal-350-want to keep at 425-450
Alk-7.3 want to keep it at 7-8

After dosing the 3 Monday, Tuesday, and testing Wednesday this is what they got to,
Mag-1400
Cal-425
Alk-9.8

I didn't dose anything Wednesday so I could see what they fell to and here is those numbers
Mag-1300
Cal-420-425
Alk-7.3

I feel like I could dose Cal at least for now on Monday and Friday and be just fine, Alk I'm still trying to figure out, but Mag seems to be dropping pretty fast by the day. My question is this, how much do I need to dose daily to keep mag at 1400? I'm using Kent Marine Mag as my Mag dosing and on the back it says to dose 1ml per gallon a day to keep it between 1250-1350 which is their recommended mag to keep the tank at. But at 20ml a day that seems a bit high? Am I right in thinking that or should I just do the 20ml and see how it goes from there? Thank you.

hijinks7 · Jan 11, 2018

So I don't think there is any magic formula. How much you dos depends on your tank size so that is a good starting mark but if you have a SPS dominate tank you are going to use Mag/Cal a lot faster then a mixed reef.

I just kept playing day by day and looking at tests to see what I had to increase and increase what I dosed per day very slowly.

Randy Holmes-Farley · Jan 11, 2018

The magnesium drop is purely testing error. The actual drop in one day is less than 1 ppm in this tank.

fishyman19 · Jan 11, 2018

Randy Holmes-Farley said:
The magnesium drop is purely testing error. The actual drop in one day is less than 1 ppm in this tank.

Not sure I follow that logic, I tested yesterday didn't dose and tested again today. I'm using a couple month old Reef Foundation Pro Testing kit and in terms of that results in ML it went from 0.70-0.65 in a day. Which is a 1400 down to a 1300 swing according to the test results sheet. I guess I'm confused then by what your meaning Randy.

Randy Holmes-Farley · Jan 11, 2018

fishyman19 said:
Not sure I follow that logic, I tested yesterday didn't dose and tested again today. I'm using a couple month old Reef Foundation Pro Testing kit and in terms of that results in ML it went from 0.70-0.65 in a day. Which is a 1400 down to a 1300 swing according to the test results sheet. I guess I'm confused then by what your meaning Randy.

It simply isn't possible for magnesium to drop 100 ppm in a day.

The logic is that there is only one way that magnesium is consumed in a reef tank, and that is by a small inclusion in calcium carbonate where the magnesium substitutes for a small amount of calcium in the crystal structure. There is NO OTHER way for it to decline (except water change with a low magnesium mix, or by lowering salinity, neither of which are consumption).

The known ratio in calcium carbonate deposited from seawater is:

2.8 dKH (1 meq/L) of alkalinity
~17.5-20 ppm calcium
~0-1.5 ppm magnesium

The amount of magnesium is variable within the range I show because some organisms allow more or less magnesium to enter the calcium carbonate in place of calcium.

But it is never substantially more than the range I show.

So if you really had a 100 ppm decline in magnesium, you'd have to simultaneously see a 187 dKH drop in alkalinity and a 1,166 ppm drop in calcium. Obviously those didn't (and cannot) happen, so magnesium did not really drop that much.

FWIW, magnesium testing errors are very common.

fishyman19 · Jan 12, 2018

Huh I didn't know that, thanks for explaining!
So would it be smart to dose mag and calcium an hour after each other so the cal doesn't take away from the mag and drop it? Or I guess, what is the best way to dose the two?

Randy Holmes-Farley · Jan 12, 2018

fishyman19 said:
Huh I didn't know that, thanks for explaining!
So would it be smart to dose mag and calcium an hour after each other so the cal doesn't take away from the mag and drop it? Or I guess, what is the best way to dose the two?

Magnesium and calcium can be mixed and/or codosed without issue. Many two part systems combine them into the calcium part.

If you are using my DIY (like BRS), the sulfate in the magnesium part cannot be mixed with calcium outside the tank (calcium sulfate will precipitate). But there's no concern about dosing them near in time to a tank.