Calcium Issues

reefer1 · Jul 14, 2015

What can cause a calcium spike in a reef tank?
Alkalinity is locked in at 130-135ppm. Calcium was around 450ppm last week but now measures 550ppm after 10% water change. Dosing is set to very low level for calcium and shouldn't raise it this much. I use Tropic Marine Pro Reef salt. (This spike seems larger than other times after water change.)
Also, why does it seem that calcium is not really consumed in the tank. It seems that only magnesium requires heavy dosing. I do 10% water changes every 10 days. The 105-gallon tank has a number of hard corals (eight 1-2" sps frags, one 5" montipora, three favia corals, and six 2" chalices).
Is this a sign of some other issue?
Thanks

Downbeach · Jul 14, 2015

What are you using to measure your SG? If its a swing arm type hydrometer, or a device that needs calibration, you may be adding too much salt mix, and by extension too much Ca. Are you sure of your test kits/proceedures? I don't know why you would have to keep dosing heavy amounts of Mg, there is very little consumed in any system. What do you consider "heavy" dosing?

twilliard · Jul 14, 2015

I was thinking the same. Heavy dosing of MG?
there is an imbalance and it it pronouncing itself

reefer1 · Jul 14, 2015

The salinity is 1.0255 and is measured using a refractometer. I checked it at the LFS and so I know it is pretty accurate
The magnesium dosing is approximately 5-6ppm/day. I say heavy because it is about 8x the alkalinity level.
Is this heavy?
What about the very low calcium consumption?

twilliard · Jul 14, 2015

reefer1 said:
The salinity is 1.0255 and is measured using a refractometer. I checked it at the LFS and so I know it is pretty accurate
The magnesium dosing is approximately 5-6ppm/day. I say heavy because it is about 8x the alkalinity level.
Is this heavy?
What about the very low calcium consumption?

How old is the tank?
its odd that you are having to dose MG at such a frequent level.
have you tried stopping all dosing and just let the WC you do keeps the levels stable?
also are you testing your new mix before adding?
something is off just need to find the source.
as for the low CA consumption its all dependant on the tank itself

Downbeach · Jul 14, 2015

The uptake for Ca and Alk in a reef system is a ratio of about 5 part of Alk(2.8dkH/50ppm) to every 2 parts of Ca(~18-20 ppm). You will see a faster decline in the alk, since there is less in reserve in your system. Basically for every 2.8dKH of alk used in our system, you will only see a use of 20 ppm of Ca. Given the margin of error with our hobby grade test kits, that 20 ppm may be hard to see. As far as your SG, I would suggest getting a decent 35ppt calibration solution and check your refracometer for accuracy.

What is your Mg reading?

reefer1 · Jul 14, 2015

Tank has been established for almost 2 years
My magnesium measures about 1350 (Salifert test kit).
Alkalinity and calcium measured using Hanna Instruments test kits
Phosphate (HI-low kit) and Nitrates measure zero
Very little to no algae growth

Randy Holmes-Farley · Jul 14, 2015

As Downbeach indicated, Aside from water changes, there's no way for magnesium demand to be much more than about 10% of the calcium demand, and both MUST be accompanied by alkalinity demand. The ratio of calcium used to alk used is about 18-20 ppm calcium for each 3 dKH.

Any other big movements in calcium or alkalinity are due to water changes, dosing, or testing error.

reefer1 · Jul 14, 2015

Okay, well then something is out of whack.
My alkalinity demand is less than 1ppm/day while the magnesium is 5-6ppm/day. It's been like this since as long as I can remember. And as I mentioned, I don't even need to dose calcium...
Here are my parameters again:
system is ultra-low-nutrient with biopellets + rowaphos running
salinity 1.025 (refractometer)
alkalinity 130-135ppm (Hanna)
calcium about 550ppm (Hanna)
magnesium 1350ppm (Salifert)

Any thoughts or ideas on what is happening?

Downbeach · Jul 14, 2015

You might want to get a second opinion on the Ca test, Hanna has had some issues with that kit in the past.

Randy Holmes-Farley · Jul 17, 2015

Your alk demand is VERY low at 1 ppm per day (0.06 dKH per day). Some salt mixes start quite high in calcium, and your expected balanced calcium demand would be less than 0.5 ppm per day, which you'd never notice.

reefer1 · Jul 17, 2015

Here's some more information.
Since the Calcium results were questions, I decided to take a water sample into my LFS yesterday evening.
Alkalinity 130-135. check.
Calcium 420. check
Nitrates undetectable
Then he checked salinity. We didn't expect any issues but just to be complete.
Salinity 1.028! What the heck? I measured 1.025 on my refractometer before collecting the water sample. I check my tank regularly and any new water.
So, it seems that my refractometer drifted/moved out of calibration. When it happened I don't know.
How does high salinity affect alkalinity and calcium consumption?

I'm currently exchanging about 1/2-gallon of tank water with Ro/Di water every 8 hours or so. I plan to check alkalinity/calcium/magnesium daily and make the necessary adjustments. Any opinions on this approach to dial back salinity?

Thanks

Randy Holmes-Farley · Jul 18, 2015

A sg of 1.028 is not very high IMO. Most of the Red Sea is higher than that and corals and fish do fine. I target 35 ppt or 1.0264.

I wouldn't expect a big impact on demand as you drop salinity, but certainly the values themselves will drop.