Why does my tank seem to use lots of magnesium, a little alkalinity, and no calcium?

[tbrown]

A little background:

I have a 125 mixed reef tank with a 40 gallon sump - I estimate about 100 gallons of water after subtracting the rock and sand displacement and my dosing estimates are pretty close to accurate based on dosing for 100 gallons.

The majority of my corals are still frags and are slowly becoming small colonies. It's stocked fairly decent with fish but not over stocked - 2 Tangs, 2 Damsels, 2 algae Blennies, 2 Halichoeres wrasses, 3 clowns, and 5 Cardinals.

Now, the interesting part:

I rarely do any water changes, I dose 10 ml of phosphates daily, nitrates are up and I'm working on lowering those, calcium stays around 480, I dose about a teaspoon of sodium bicarbonate every other week, and I dose 20 ppm of magnesium every other day.

I'm guessing part of my "issue" could be that my top off water is tap water (I know...) And that it's probably high in calcium and it definitely has alkalinity (I think 7.8 dKh) so I'm technically dosing alkalinity there and probably calcium as well.

Lately though, I'm having really aggressive growth on Cyphastreas and Acros and I still am not seeing a drop.

July 4th to today:

May 29th to July 4th to today:

May 29th to today:.

No before pics but I just moved this 2 weeks ago and it's already encrusting the glass:

Again, no progress pics but both have expanded upwards and downwards by almost 1" in the last 2-3 weeks:

I'd say my tank is not suffering so the calcium must be coming in through the top off water?

 

[Rugops]

Do you have a lot of coraline algae?

 

[tbrown]

Do you have a lot of coraline algae?

Yes.

 

[Lavey29]

Magnesium is only depleted at 1/10th the rate of alk and calcium depletion.

 

[Rugops]

Yes.

I dose 5-7 ml of Mg a day. My tank is a bit less than 39 gallons. If you have a lot of rock, you are going to have a lot of growth which uses up the elements a lot faster than one would like.

Magnesium is only depleted at 1/10th the rate of alk and calcium depletion.

Depends. There are factors that change this statement. OP says that they may have lots of coralline algae

The coralline algae is a substantial sink for calcium and alkalinity (and magnesium) in reef tanks, but generally in a balanced fashion (maybe slightly less calcium use than usual as it incorporates a lot of magnesium.

 

[tbrown]

I do notice if I go a few days without adding magnesium then add a little extra to make up the coralline growth seems to jump a bit.

 

[tbrown]

I just realized I never added a FTS.

 

[Kmst80]

I just realized I never added a FTS.

I got the same in my tank. I dose magnesium heavier than any other element and I do use ro water. Got a lot of coralline too, all rock in my tank is live rock and came with coralline.

 

[Lavey29]

I dose 5-7 ml of Mg a day. My tank is a bit less than 39 gallons. If you have a lot of rock, you are going to have a lot of growth which uses up the elements a lot faster than one would like.

Depends. There are factors that change this statement. OP says that they may have lots of coralline algae

I have a ton more coraline then what's in the OP tank pic along with 38 SPS colonies and 40 other corals. My mag depletes very slowly but I dose 35 ml of alk and 35 ml of calcium daily. I am guessing some improper testing probably so maybe he should get a confirmation second test done somewhere.

 

[Rugops]

I have a ton more coraline then what's in the OP tank pic along with 38 SPS colonies and 40 other corals. My mag depletes very slowly but I dose 35 ml of alk and 35 ml of calcium daily. I am guessing some improper testing probably so maybe he should get a confirmation second test done somewhere.

Agreed. I suppose consumption differs from tank to tank. Odd.

 

[Randy Holmes-Farley]

It is simply impossible to either use 20 ppm of magnesium per day, or to use any significant magnesium in an ordinary reef tank without using alkalinity and calcium.

Even coralline uses them in a ratio of about 2.8 dKH of alk, 18-19 ppm calcium, and 1-2 ppm magnesium.

 

[Lavey29]

Agreed. I suppose consumption differs from tank to tank. Odd.

That's why all our tanks are unique I suppose and although there are sets of standards to follow, what works for me may not work for you or the next reefer. It's a really challenging hobby.

 

[Pod_01]

and I dose 20 ppm of magnesium every other day.

You are not the only person to observe this. I read it at few places and heard it mentioned in a video (don’t ask for a source).
The symptoms is that suddenly Magnesium is consumed at elevated rate and abruptly things go back to normal.
There is no explanation and it makes no sense from reef chemistry or biology.
I observed something similar early in my reefing trials but I figured I was newb and messed up the test or something else.
Regardless interesting phenomena.

 

[Randy Holmes-Farley]

You are not the only person to observe this. I read it at few places and heard it mentioned in a video (don’t ask for a source).
The symptoms is that suddenly Magnesium is consumed at elevated rate and abruptly things go back to normal.
There is no explanation and it makes no sense from reef chemistry or biology.
I observed something similar early in my reefing trials but I figured I was newb and messed up the test or something else.
Regardless interesting phenomena.

Given the terrible track record of magnesium test kits, I’d bet against big odds that such reports are either test errors or salinity changes. There’s just not a way for a cell to store large amounts of magnesium and not alk.

 

[tbrown]

I have a ton more coraline then what's in the OP tank pic along with 38 SPS colonies and 40 other corals. My mag depletes very slowly but I dose 35 ml of alk and 35 ml of calcium daily. I am guessing some improper testing probably so maybe he should get a confirmation second test done somewhere.

I've gotten confirmation. I'm guessing that it probably is then due to the top off water being high in calcium and alkalinity I don't see th same drops.

It is simply impossible to either use 20 ppm of magnesium per day, or to use any significant magnesium in an ordinary reef tank without using alkalinity and calcium.

Even coralline uses them in a ratio of about 2.8 dKH of alk, 18-19 ppm calcium, and 1-2 ppm magnesium.

It's 20 every other day, not daily so only 10 ppm. So then it's most likely due to the top off water? High alkalinity and calcium offsetting the demand, but low magnesium not keeping up?

 

[tbrown]

Given the terrible track record of magnesium test kits, I’d bet against big odds that such reports are either test errors or salinity changes. There’s just not a way for a cell to store large amounts of magnesium and not alk.

My magnesium will hover at 1260-1280 and not seem to deplete much but when I get it to 1350 or higher is when I start to see the drop.

If it helps, I keep my alkalinity at 8.3 dKh and I've not had to dose calcium since setting this tank up.

 

[Pod_01]

Given the terrible track record of magnesium test kits, I’d bet against big odds that such reports are either test errors or salinity changes. There’s just not a way for a cell to store large amounts of magnesium and not alk.

Randy you likely got the two main causes.
Reflecting back I suspect in my case in the early days it was combination of incorrect salinity and test error. I struggled a lot with getting consistent salinity measurement, the salinity probe was up and down and refractometer gave me 5 different readings in a row.

 

[JoJosReef]

@tbrown reaches in the fridge for the coral snow jug, grabs the milk jug. And you know he's humming "It's beginning to look a lot like Christmas", while he's pouring it in his tank.

 

[Randy Holmes-Farley]

I've gotten confirmation. I'm guessing that it probably is then due to the top off water being high in calcium and alkalinity I don't see th same drops.

It's 20 every other day, not daily so only 10 ppm. So then it's most likely due to the top off water? High alkalinity and calcium offsetting the demand, but low magnesium not keeping up?

No, there’s no scenario where a reef tank uses 10 ppm of magnesium per day.

Any efforts to explain it have to involve something other than consumption, such as test error or salinity changes.

 

[Randy Holmes-Farley]

I've gotten confirmation. I'm guessing that it probably is then due to the top off water being high in calcium and alkalinity I don't see th same drops.

It's 20 every other day, not daily so only 10 ppm. So then it's most likely due to the top off water? High alkalinity and calcium offsetting the demand, but low magnesium not keeping up?

What are you topping off with?