Why Mg keeps dropping?

Chris C · Jan 27, 2016

Last week, I realised the Mg was at 1100 after a 20% WC. So I dosed Epsom salt and brought it up to 1170. Today, 4 days later, I tested it again and it's at 1125. Alk and Ca have been stable at 9 dkH and 400ppm respectively. What's going on in my tank??

CastAway · Jan 27, 2016

Do you dose two part, or run a Ca reactor?

joshkirkland83 · Jan 27, 2016

This is a chart I use to keep stable water parameters.

Chris C · Jan 28, 2016

@CastAway I dose two part.

@joshkirkland83 thanks!

Randy Holmes-Farley · Jan 28, 2016

It is likely that nothing special is going on in your tank. The results, at least some of them, are test error. There is no mechanism at all in a reef tank that allows magnesium to drop 10 ppm per day. Don't add any more magnesium until we get a handle on the testing. Magnesium might be low, but it also might not be.

What kit are you using? A number of folks seem to get low readings with the Red Sea kit.

What salt mix are you using?

You might try the kit on some new salt water.

Chris C · Jan 28, 2016

I use API testing kit and AB Reef salt. Both are quite basic

Randy Holmes-Farley · Jan 28, 2016

Let's see what you get on some new salt water. I'm not familiar with AB reef salt to know if it might be low.

Also, what is your salinity and how are you measuring that? Low salinity is a common cause of low magnesium.

Chris C · Jan 28, 2016

Salinity is at 1.025 and I measure it with a refractometer

Jimbo662 · Jan 28, 2016

Are you seeing an increase in coralline growing?

Randy Holmes-Farley · Jan 28, 2016

Chris C said:
Salinity is at 1.025 and I measure it with a refractometer

How did you calibrate it?

FWIW, raising the salinity to 35 ppt (sg = 1.0264) will boost the magnesium 1170 ppm to 1236 ppm.

Chris C · Jan 28, 2016

The refractometer is only 2 months old. How often should I calibrate it?

Jimbo662 said:
Are you seeing an increase in coralline growing?

Coralline algae always grow. In fact, it went from light purple to solid dark purple to maroon to blood red and recently I've seen brown ones.

Randy Holmes-Farley · Jan 28, 2016

Chris C said:
The refractometer is only 2 months old. How often should I calibrate it?

Now would be a good time.

Some folks calibrate it once a week or more. How often is necessary depends on how fast yours seems to get out of calibration. It may be fine, but it never hurts to test again, especially when anything seems amiss that might relate to salinity.

.

Chris C · Jan 28, 2016

Alright. How do I do a proper calibration? Using distilled water?

Randy Holmes-Farley · Jan 28, 2016

Best is with a 35 ppt standard, unless it is a digital refractometer (like the Milwaukee) that forces you to use pure fresh water.

If it is a true seawater refractometer (most are not unless they state so), the RO/DI can be used, but the 35 ppt standard can be better (assuming the standard is accurately made).

I discuss refractometers and calibration here:

http://www.reefedition.com/refractometers-salinity-measurement/