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Epsom Salts = Magnesium


First off: Never add anything you don't test for! -Arm thyself with a good Mg test kit.

Seachem's Magnesium Test Kit- gets you 75 tests for about $48 and includes an 1180 reference mix.If you find a better kit out there for Mg - let us all know (after you've used it).

Initial test: Find out how bad your Salt Mix is on Mg. Mix a fresh batch to 35 ppt, --> Aerate till PH is up to 8.2 --> run Mg and Ca tests on this batch and record the result.

Use this as a reference point on where you are starting off at. Remember - each time you do a water change with a Salt Mix that has only 950 ppm Mg you are "hurting" your Mg levels in your tank. I know - it bites... but every Salt manufacture is doing the same thing and then selling us "additives" to make up for it.

The Epsom salts quickly pays for itself at 1.89 for a 4.4 lb (2 Kilograms) bag. Now - since our favorite salt mix (because of the other things it DOES contain in NSW concentrations) happens to read 950ppm Mg. We only need to put about 1lb into the RO top off every time we do a 30 gal water change to keep our Mg at 1350.

Useful tool:

I found a Reef Chemistry Calculatorwebsite --> this calculator lets you figure out how much of a Mg product to add per gallon to raise the Mg from your starting concentration (whatever your testing showed) up to the recommended endpoint of 1350 ppm. The calculator is very easy to use, and (it works out the same for Epsom Salt quantities per our testing here). Go conservative a bit. And test and tune to suit. (This is why getting a 75 test kit was worth it).

Note: do not try to add MgSO4 crystals directly to mixed salt water--> it's already super saturated, and you may force the water to precipitate it right back out of solution..

Instead - try to mix it with pure RO in your top-off water. Don't raise Mg levels too fast .no more than 100 ppm/day.

Final Note: raising Mg in a your system using this method may raise your refractometer reading form 35ppt to 36ppt but don't panic - I have seen it drop back to 35 ppt as the week progresses and as evaporation gets replaced with RO once you stop adding MgSO4. You may notice you don't need nearly as many CaCO3 additives once Mg reaches NSW level. I have saved $40/month in additives by getting our Mg right. (I do run a calcium reactor constantly though). I haven't had to add any other additives (which I used to have to do pretty constantly before we tested/remedied our Mg problem).

What's bizarre is how much the montipora growth rate has shot up since doing. I never thought possible to see these growth rates (without tons of additives) in a home tank.

The way I see it we have three choices in the world for adding Mg

MgSO4 (magnesium sulfate)

MgCl2 (magnesium chloride)

MgO (magnesium oxide)

Of the three, I don't like add chloride, as there's enough sodium chloride in the system already (salt) and the precipitate release is chlorine. The oxide one is very unstable, and reacts too violently (won't stay in solution long enough to be useful), so that really only leaves the sulfur choice as practical. H2S is a natural endpoint of the anaerobic bacteria process (later turns to DMS before released into atmosphere) - since this process is prevalent in the oceans already I feel safer introducing an excess of sulfur atoms into the system than an excess of chlorine atoms.

The brand of Epsom salts is not really as important as the purity. So, just check the label make sure it's USP (United States Pharmacopeia (USP), This means you are getting the purity that's pretty much regulated and monitored by some type of an authority - and this one is for standards for human consumption which I can pretty much guarantee carries much more legal weight than technical grade substances (like road de-icer salts - Magnesium Chloride). Do some of your own Google research using keywords "Epsom salts FDA" and you will hit some goods links with PDF files which are MSDS sheets from a company called PQ Corporation which is one manufacturer of both Technical Grade, and USP (Pharmaceutical Grade). Any Pharmacy which sells "Epsom Salt, Magnesium Sulfate, U.S.P." should be suitable. (We get ours at HEB pharmacy - just because that's convenient for us).

I have read the chemical content labels on one Manufacturers' brand (will remain nameless here, since this is a public forum): and I noted the Mg additive contains 2 types of Mg salts: Magnesium Sulfate, and Magnesium Chloride. The interesting thing is - they don't give amounts like 50/50 or 75/25, or what... so for all I can tell, this could be 99% Epsom Salts (sulfate) and 1% Road de-icer (chloride). There's no way of telling what they are doing, really.

At least with the Pharmaceutical grade stuff, we know the purity of what we are getting - check the labels. Most USP labels I see show guaranteed analysis of 9.8 percent water soluble Magnesium, and 12.9 percent Sulfur. Each molecule contains 1 sulfur Atom, 1 Magnesium atom, and 4 Oxygen atoms. (For your weight proportions),

so - yes you are buying a lot of oxygen..(in the salt).

Again - use the calculator on the Reef Chemistry Calculator

for the amount needed to adjust per gallon. And don't raise it more than 100 ppm/day. <-- This is important!

It wouldn't matter if you were using a Named Brand Magnesium additive, or Epsom Salts USP, as far as the calculations go. - Just do the changes slowly, because each day it raises, organisms are going to react. (Some things will start new growth spurts - but this means they will start consuming Calcium and Magnesium and Strontium at an even faster rate than before). --But don't worry, this is usually what we stony growers are after anyways, - just be aware that you are going to be altering the water chemistry, and don't shock them - let them adjust. It also takes time for the water to balance so I would wait 6 hrs after adding to re-test for Mg/Ca/KH levels.

Do you run Carbon? – Yes. This is to remove any impurities in the additives (we don't trust the industry "completely" on the purity thing, since nobody's watching (regulating) them like USP FDA., etc. The purity of the Carbon is also just as important. For all we know - someone could be bottling USP grade chemicals, and selling them, OR they could just as easily be bottling technical grade chemicals (purity in question). And - it's the impurities we wonder (worry) about.

What is the effect so adding Magnesium?

"A 100 ppm addition to magnesium with any brand of additive (except maybe SeaChem which may be higher due to their unusual recipe) will add about 0.4 ppt to the salinity, so 35 ppt to 35.4 ppt, or sg 1.0264 to 1.0267"

One last note, bringing your mag up too fast will shock your SPS. It may cause burning of the tips, dead spots and browning. Long term use may affect ionic balance if no water changes are made. Limit the increase to a Max. of 100 ppm/Day.

Information gleaned from discussions with cmanning.

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