why does my magnesium drop?

fcbarca9lona · Nov 1, 2014

hey guys this is my first post on here. i always watch the vivd aquariums videos on youtube and i enjoy everyone of them. but i have been having little problem with my water parameters. im currently doing the red sea coral growth program and following their recommended water parameters. and it asks for the alk to be at 12.0 for lps coral growth but for some reason evrytime i check my water the alk doesnt go higher then 9 and everytime that i dose alk buffer my magnesium drops. im using the red sea test kits for everything apart from the ammonia, nitrites and ph tests.
my tank is 150g mainly lps with couple of zoas and mushrooms.
okay here are my water test from the last 3 water tests.

oct 23 2014
temp-82
ammonia- 0
nitrates-0.5ppm
nitrites-0ppm
ph-8.0
alkalinity-8.4
salintiy-1.024
calcium -410
phosphate-0.08
magnesium-1310

oct 27 2014
temp-83.3
ammonia- 0
nitrates-0.25ppm
nitrites-0
ph-8.0
alkalinity-7.6
salintiy-1.025
calcium -450
phosphate-0.08
magnesium-1310

oct 31 2014
temp-81.0
ammonia- 0
nitrates-1ppm
nitrites-0
ph-8.0
alkalinity-8.4
salintiy- 1.025
calcium -470
phosphate-0.16
magnesium-1240

idk what else you guys would need to ask me about my tank but feel free. and thanks to whoever takes they're time to reply.

Randy Holmes-Farley · Nov 1, 2014

Unless you did a water change, magnesium did not really drop by 70 ppm in 4 days.

The actual depletion rate for magnesium, assuming no water changes, is more like 0.1 to 1 ppm per day or a bit more, and is usually less than or equal to 1/10th of the depletion rate for calcium since it is incorporated into calcium carbonate at about that rate.

There is no other big sink for magnesium in reef aquaria.

Randy Holmes-Farley · Nov 1, 2014

These have more on why magnesium does very slowly decline if not added to reef aquaria:

Aquarium Chemistry: Magnesium In Reef Aquaria ? Advanced Aquarist | Aquarist Magazine and Blog

Do-It-Yourself Magnesium Supplements for the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com

fcbarca9lona · Nov 1, 2014

well could it be that trying to buffer the alk is making the magnesium drop? i added the recommended buffer for the alk but i didnt see the alk raise and i didnt test the next day to see it either, but everytime that i try to get the alk to 12 my magnesium drops

Randy Holmes-Farley · Nov 1, 2014

Nope, there is nothing you could possibly have done in four days to drop magnesium by 70 ppm. It is testing error (or a water change).

Even if somehow you could precipitate only magnesium carbonate and no calcium carbonate (which doesn't happen), you'd need to have added the equivalent of 16 dKH worth of alkalinity. Since magnesium only comprises about 10% of the calcium in such precipitates, you'd need to have added more than 100 dKH worth of alkalinity and would see massive precipitation throughout the tank.

That said, why are you trying to get the alkalinity to 12 dKH? That is too high, IMO, for a target.

I recommend 7-11 dKH.

ReeferEric · Nov 2, 2014

Check with another test kit. I find it hard to believe that it can drop so quick unless you have an established tank with thriving corals and lots of coralline algae.

Frozen Tundra Reef Dreams · Oct 16, 2022

why would my magnesium drop like this?

it is at 1350 so I add some BRS magnesium maintenance. next trident test is at 1465. two days later it is 1399.

Alk 8.65
Ca 550

Im also trying to lower my calcium. I do trident controlled dosing.

what gives?

mdb_talon · Oct 16, 2022

Frozen Tundra Reef Dreams said:
why would my magnesium drop like this?

it is at 1350 so I add some BRS magnesium maintenance. next trident test is at 1465. two days later it is 1399.

Alk 8.65
Ca 550

Im also trying to lower my calcium. I do trident controlled dosing.

what gives?

Incorrect test results. My bet would be the 1465 was erroneous.

Randy Holmes-Farley · Oct 17, 2022

The only possible ways to get rapid changes in magnesium are dosing (for rises), salinity changes, or test error.

Magnesium does not ever detectably rise on its own, and real magnesium consumption is 0-2 ppm per day, and is typically 1/10th of the calcium consumption or less.

Frozen Tundra Reef Dreams · Oct 17, 2022

mdb_talon said:
Incorrect test results. My bet would be the 1465 was erroneous.

i did dose a cup of the magnesium mix which would suggest a rapid rise, maybe before it was dissolved into solution thoroughly enough the trident tested a section of water that was high, then more thoroughly mixed after? I dosed in aquarium while tridenttakes water from sump.