What causes low Mg?

MarlinHooker

MarlinHooker

Member
I have a 150g mixed reef tank that has been setup for about 3 months now. Alk has always been low (~ 6.67 Dkh) & Ph has been a little on the low side as well (7.95). All other parameters show good (Ca 500, Phos 0, nitrate 0, ammonia 0, SG 1.025, temp 79 controlled by a chiller). I finally got my Mg test kit & checked it last night and no real surprise, it is reading low (1125). So, now I know why I haven't been able to get my Alk up even though I've been dosing but my question is what would cause low Mg?

I use Red Coral Sea Pro salt and do a 15g PWC weekly ...
 
Are you dosing any trace elements? alkalinity, calcium, or magnesium? Besides water change with coral pro...

Generally speaking high alkalinity would cause precipitation of Trace elements... Which is not the this scenario...

What type of coral are you housing and amount of coralline algae growth?
 
1125 ppm of Mg is not low. It's an okay measurement. I would still recommend getting it up to the 1300-1400 ppm range just for that extra safety net. Corals and coralline do consume a small amount of Mg so it's important to check it once in awhile. It will not drop nearly as quickly as Ca or Alk but will drop over time regardless.

Alkalinity is often the first one to get depleted because there's a very small amount of it in the water column. It can go surprisingly quickly - it's not unusual for it to drop 1.5 to 2.0 dKH and in some cases more per day.
 
There are several types of aquariums that utilize Mg in different amounts. In fact all tanks will vary to some degree. Mg is an important player in the bio processes of marine critters. Mg is a major not trace element in the aquarium and is related to all bio processes including skeletal building. One cannot maintain proper levels of Ca or Alk without proper levels of Mg. Normal levels are considered to be between 1250-1350ppm. Less than that can cause precipitation of carbonates and calcium. Personally I have found this can be true for levels below 1180ppm when trying to hold Alk at 9 -10 dKH range and Ca in the 450ppm range.

Now to specifically answer your question of what causes low Mg. Although quality salt mixes do provide adequate amounts of Mg for a FOWLR tank, not all mixes are equal. So if your mix is on the low side of the spectrum then addition of Mg may be needed for higher functioning reef tanks. This is because any tank holding critters that require additional amounts of Ca and Alk will also utilize additional amounts of Mg. And maintaining live rock in large amounts along with fast growing LPS and SPS corals will create the need to supplement Mg as it is used up in the skeletal growth of the inverts. Even hobbyist who keeps only a few LPS with mostly soft corals in a live rock tank will normally need to at least boost elements with an all in one type supplement such as Salifert All In One or Seachem Complete. So water changes are not enough to keep up with the demand from inverts is the normal reason for Mg depletion and low levels. If you’re testing then you should be dosing when below 1250ppm.

Merry Skerry
 
What is your salinity? What are you measuring it with? Have you checked the readings of your make up water?
If you're using a hydrometer, it could be giving you an inaccurate reading, and you may not be adding enough salt mix, which, by extension you are not adding enough Mg either.
I would get/use a calibrated refractometer.
 
I'll try to answer everyone's questions:

1. I have been dosing Alk with no change, it stays pretty constant at ~ 6.67 Dkh. Ca has remaining steady at 500 without dosing. No real coralline growth at this point. Inhabitents are 9 sm fish, 2 sm Zoa frags, 2 chaice frags, acan frag, 4 sm monti cap frags, 1 digi cap (3"), 1 lg frogspawn, 1 med hammer, 1 plate coral, 1 lobo and 1 lg open brain, 1 med rose bubbletip, 2 cleaner shrimp. most of the larger pieces were recently added and the low Alk has been there since the beginning
2. As I mentioned I use Red Coral Sea Coral Pro salt which claims to be 1400 Mg. Unfortunately since I was only able to check Mg for the first time last night I don't know if its depletion is a recent event due to all the new corals in the system or not. I suspect not since my Alk has always been low despite dosing Alk.
3. Also as mentioned in my first post, SG is 1.025 & never varies. I have an ATO system which does not allow hardly any evaporation before adding RO/DI water. I use a Milwaulkee digital refractometer to measure with as well. I have a 75g/25g water mixing system where I make up 25g of fresh seawater from my 75g RO/DI storage tank which recirculates for 48-72 hrs prior to a PWC.
I also have a Geo 618 Ca reactor which I'm getting ready to setup but I need Ca/Alk/Mg stable at the right levels before turning on the system ...
 
Maybe make a new batch of salt water and measure the parameter in the new water?

With 150 gallon and weekly water change, I don't think it would drop that quick with your stock list in 3 months.
 
I agree that 1125 ppm isn't all that low, and won't prevent you from dosing up the alkalinity. I ran tanks at 1100 ppm for years without trouble, because that was the magnesium level for Instant Ocean. That said, I'd likely raise the level to the canonical ocean average of 1285 ppm.

I'd check some freshly-mixed saltwater. If that measures the same as the tank, that's probably the issue. It'll take a lot of supplement to raise the level, though, and you might want to get a second opinion on the kit before investing a lot of money.

To raise the alkalinity, you'll just need to dose more. Many tanks consume 2-3 dKH per day, even with no stony corals in the tank. Baking soda is a cheap way to go.
 
A lot of good info here

That Red Sea coral pro salt has proved to be a problem for me. I have tested fresh batches to be inconsistent...I was instructed that hobbyists need too mix the salt well prior to mix and that it needs to be slowly added to high flow water. Which I have been doing for over a year, as i got morw anal i started testing and the results were constantly different and had multiple readings of off the chart alk - with multiple kits (I ordered two sets to make sure It wasn't me and have had friends test for me)

On my nano it would have been a big problem because it would have caused a large swing, and it did - multiple times killing various acros.

All tanks consumption are different.. But
Personally I don't see how you could be getting those readings at 1025 without dosing, bad test kits, or it being a bad batch of salt.

Have you tried multiple test kits?

Like i said having 500 calcium with 11xx mag and 6 alk just doesn't add up if your not dosing. Jmo (did I miss something?)


Anyways theres a few options -

Doing a few rather large water changes to get your tank in line (with tested water)

Or

You can dose both mag and alk until it raises to an acceptable range and retain those levels while allowing your calcium to drop.

Please correct me if I'm wrong.

-jonny
 
Ok, first I'll check the fresh batch of seawater to see how it compares. As for test kits I have the Hanna colorimeters for both Alk & Ca and the Salifert kit for Mg. I'll see about asking a buddy to come over with his kits to see what he gets.

I'm still new to the corals thing ... I've been in the hobby a long time but there has been a prolonged absence before this new tank. Last time my tanks were up and running no one had a reef tank with corals in them ...

that said, it does seem strange to me that Ca has always remained the same while Alk & Mg & Ph are on the low side. Since I just became able to test Mg 2 days ago I had been trying to understand why Alk was at 6.67, Ph was ~ 7.9 ish. I was orginally using Tropic Marin Pro salt & was convinced to switch because of the low Alk/Ca levels. Since switching I've gone through 1 bucket of Red Sea Coral Pro salt. I did dose Alk for a short time and while it got my Ph up slightly (it had been as low as 7.8-7.85) Alk did not rise appreciably. i didn't realize Alk could drop as much as you all are saying. I have plenty of baking soda on hand so I can start dosing that daily to see if I can raise Alk up to around 8 Dkh.

I don't want to attack too many of these at the same time as I do understand they affect each other ... that being said, what would you all recommend as a course of action given my current state?

I could certainly switch salts? if so, what would you recommend?
If a fresh batch of Red Sea tests out as advertised (high Ca/Alk/Mg) how would you go about getting acceptable levels?

Am I correct in assuming that once I have the system at a set of good numbers and get my Ca reactor up and running & dialed in that my Ca/Alk/Ph issues should stay in line and all I might need to dose would be Mg?
 
I'll have a friend of mine check with his kits. I use the hanna colorimeters for Ca & Alk and salifert for Mg. Since I only recently am able to check Mg my focus has been on my low Alk & Ph. I dosed baking soda for a few days & the Ph raised to about 7.9 ish but Alk never got to 8 Dkh. Sounds like zI may hvae not been doing it long enough ... Ca has always shown 500 which may mean I have testing issues ...

So, if a new batch of Red Sea Coral Pro checks out with advertised Alk/Ca/Mg its ok to dose Alk & Mg at the same time?

I orginally was using Tropic Marin Pro but was convinced to switch to address my issues because of the low numbers. I've gone through 1 bucket (175g) of the RSCP salt. If the fresh salt numbers aren't goo I could switch salts but what would you all recommend I go with?

Also, am I correct in assuming that once I can get stable acceptable numbers for Ca/Alk/Ph/Mg & have my Ca reactor up and running & dialed in I would not have to dose anything accept for possibly Mg?
 
Hanna has had some problems with their Ca meter, so it would be a good idea to have that checked. Ask your friend if he has a calibrated refractometer to double check your salinity in both your make up water and DT.
 
ok, checked a fresh batch of Red Sea salt and it was 1280 Mg, 7.8 Dkh Alk and 514 Ca although still don't know about my Hanna Ca colorimeter.

Been dosing baking soda and tank is now at 7.5 Dkh and Mg was at 1100, guess I need to add more Mg in each dose as i was only adding 50 ml last night. I'll get my friend's Ca kit tonight to see what's going on there ...
 
Yeah I know, I was just to dump a bunch in all at once. I know it will be dependent on my coral load but if i can keep Alk and Ca stable about where they should be, how quickly does Mg get depleted, in other words, do I need to worry that my depletion rate is such that I'll have dose dose extreme amounts to make up for normal usage and get the level back to 'normal'?
 
I never needed to dose magnesium at all, except when I forgot to bump up a batch of the old IO when I was doing water changes. The magnesium consumption is hard to predict, but I suspect most tanks are okay with the level being measured once every month or two.
 
Ok, thx. No real sign of stress with everything in the tank even though Mg has probably always been around 1110-1150 so I won't get too excited about getting it up too quickly ...
 
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