Magnesium never depletes....ever.

willthethrill · May 17, 2017

I've been dosing alk and cal for about 8 months now. About 100ml each per day. Maintain them around 9.0 and 435. I do weekly water tests, 20% monthly water changes. I only dosed magnesium the initial week to get the level right, and ever since then (8 months), every time I test, magnesium is always the same and never drops. What gives? (I've used two magnesium test kits to double check)

jsker · May 17, 2017

willthethrill said:
I've been dosing alk and cal for about 8 months now. About 100ml each per day. Maintain them around 9.0 and 435. I do weekly water tests, 20% monthly water changes. I only dosed magnesium the initial week to get the level right, and ever since then (8 months), every time I test, magnesium is always the same and never drops. What gives? (I've used two magnesium test kits to double check)

A lot of salts are out there are mixed with high magnesium, and doing a 20% water change every week is going to keep the Mg high.
What is high??

ritter6788 · May 17, 2017

I had a full blown sps reef for years and never added a drop of magnesium.

Jlentz · May 18, 2017

Same here. It never goes down.

Mindi · May 18, 2017

Yep me too...have not added Mg in 7 years

Randy Holmes-Farley · May 18, 2017

Actual magnesium consumption is very low. About 1/10th of the calcium depletion rate, or less (depending on the organisms doing the consuming).

As folks have said, the elevated magnesium levels in many salt mixes can sometimes offset the demand.

That said, if you are using a DIY two part, i recommend using the magnesium part at the recommended dose since it does more than maintain magnesium, and shouldn't cause it to increase but rather maintain it roughly where it is.

willthethrill · May 18, 2017

Randy Holmes-Farley said:
Actual magnesium consumption is very low. About 1/10th of the calcium depletion rate, or less (depending on the organisms doing the consuming).

As folks have said, the elevated magnesium levels in many salt mixes can sometimes offset the demand.

That said, if you are using a DIY two part, i recommend using the magnesium part at the recommended dose since it does more than maintain magnesium, and shouldn't cause it to increase but rather maintain it roughly where it is.

I use the BRS two part. Also bought 1/2 gallon of the BRS magnesium mix powders, but never had to dose it beyond the first week when I brought it up to 1350. Ever since then whenever I test it's always around 1400. Even at 1/10th calcium rate like u say, Which im using 100 ml a day, I don't know that the 20% monthly water change is adding in all that mag to keep it at 1400. This has been an aspect of my tank that's had me confused for a long time.

Randy Holmes-Farley · May 18, 2017

Some folks get unusually high magnesium readings from the Red Sea kit. Make sure that isn't the issue.

I'm not sure what size your tank is, but assuming it is 100 gallons for calculations, 100 mL a day of the DIY (like BRS sells) is only adding 10 ppm per day of calcium.

My recipe calls for adding the equivalent of about 2 ppm of magnesium per day in that scenario. 1 ppm of that is to offset demand, and 1 ppm of that is to correct for changes in salinity from adding the two part and then reducing the salinity back to normal levels. That's 60 ppm per month.

Even with zero demand, you will lost about 30 ppm magnesium per month due to the salinity rise effect from the two part.

I expect it takes a considerable period to determine exactly what is happening with magnesium since most kits can hardly see +/- 30 ppm magnesium, and water changes with a mix that is above the tank level will reduce that apparent demand. I don't know the details of magneisum in your mix and tank, but here's a graph of potential impact on magnesium under certain scenarios:

Water Changes in Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2005-10/rhf/index.php

from it:

Figure 20. A blowup of Figure 19 to allow comparison of different water change rates on magnesium depletion when using a moderate amount of calcium chloride and sodium bicarbonate to maintain calcium and alkalinity.

willthethrill · May 20, 2017

Randy Holmes-Farley said:
Some folks get unusually high magnesium readings from the Red Sea kit. Make sure that isn't the issue.

I'm not sure what size your tank is, but assuming it is 100 gallons for calculations, 100 mL a day of the DIY (like BRS sells) is only adding 10 ppm per day of calcium.

My recipe calls for adding the equivalent of about 2 ppm of magnesium per day in that scenario. 1 ppm of that is to offset demand, and 1 ppm of that is to correct for changes in salinity from adding the two part and then reducing the salinity back to normal levels. That's 60 ppm per month.

Even with zero demand, you will lost about 30 ppm magnesium per month due to the salinity rise effect from the two part.

I expect it takes a considerable period to determine exactly what is happening with magnesium since most kits can hardly see +/- 30 ppm magnesium, and water changes with a mix that is above the tank level will reduce that apparent demand. I don't know the details of magneisum in your mix and tank, but here's a graph of potential impact on magnesium under certain scenarios:

Water Changes in Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2005-10/rhf/index.php

from it:

Figure 20. A blowup of Figure 19 to allow comparison of different water change rates on magnesium depletion when using a moderate amount of calcium chloride and sodium bicarbonate to maintain calcium and alkalinity.

My total volume is 160g. I used two different salifert test kits. And I use Red Sea pro salt.
Thanks for all that great info. But it still leaves me stumped because that information is telling me magnesium is depleted over time. On that chart with the water change percentages over time, magnesium is still slowly dropping. And I have not used a DROP of magnesium in 8 months, yet it's always reading 1380-1400. From only 20 % monthly water changes. Also weird how some of the guys who also commented said they had full blown reef tanks and hadn't dosed magnesium in years. [emoji849]

Medic755 · May 20, 2017

Have you tested your water after you mix it?

Randy Holmes-Farley · May 20, 2017

willthethrill said:
My total volume is 160g. I used two different salifert test kits. And I use Red Sea pro salt.
Thanks for all that great info. But it still leaves me stumped because that information is telling me magnesium is depleted over time. On that chart with the water change percentages over time, magnesium is still slowly dropping. And I have not used a DROP of magnesium in 8 months, yet it's always reading 1380-1400. From only 20 % monthly water changes. Also weird how some of the guys who also commented said they had full blown reef tanks and hadn't dosed magnesium in years. [emoji849]

Most likely, the depletion is too small for you to detect with hobby kits, which are very noisy.

There is just ZERO possibility that there is no depletion. Simple addition of calcium chloride and sodium bicarbonate/carbonate, combined with proper salinity maintenance, cannot possibly ever result in no depletion even if nothing in the tank consumes any. But with many kits, +/- 20 pm magnesium is doing a good job. Some are way worse.