Magnesium low - need advice

[JJ70]

So the tank is approx 5 months old and my magnesium levels are low and I'm having difficulty bringing them up. I dose Kalk in the topoff and run GFO in a reactor. I also have been dosing Mag daily (ESV). I perform 20% wc every two weeks using IO salt.

Tank:
SG: 35 PPT (refractometer- calibrated before each wc)
TEMP: 78-79 F
PH: 8.2-8.4
Alk: 10.2 (Sailfert)
Cal: 360 (Sailfert)
Mag: 980 (Sailfert)
Nitrate: 4 ppm
Phos: 0 ppm
Flow: Tunze wavebox 6208, tunze 6105 and Jebao RW-8

I have tons of coraline growing as well as a birdnest and one monitpora along with several zoa colonies and a few LPS that are all doing well. I have some decent size macro growing in the DT (dragon flame and some other unknown red macro)

The tank has 2-3" of Carib Sea special grade sand that I have noticed is solidifying in a few spots within the tank even though I have 20-25 Ceriths and 25-30 nassarius snails moving throughout the sand bed.

Any ideas on why the mag is so low. I have already gone through a 32 oz bottle of the ESV mag product?

Thanks!

[downbeach]

What are you using to calibrate your refractometer? If its plain RO/DI water it may be off, causing you to add too little salt mix to your make up water, and by extension too little Ca, Mg, etc.

[JJ70]

Thanks for the response. I use 35 PPT calibration solution. I actually ordered additional calibration solution from another supplier because I thought that may have been the problem too but they are both the same.

[jbambas]

What about a bad lot of salt? May be well worth it to switch out salt, even just a single bag of another brand. Honestly, who knows what type of quaity control goes on at any of these companies. The salinity may be right but the addatives in the salt mix itself could be completely off. Something else I know is that salt can and does separate. Bags and buckets should both be mixed prior to using them. Your mag could be low today and a complete overdose next change.

Just my 2 cents,

Jeff B

[bertoni]

Hmm, I get about 27 fl oz to raise the magnesium level by 100 ppm, assuming about 80g of actual water in the system.

Usually, when the magnesium level is that low, the issue is the salt mix. You might have gotten a bad batch or the dry salt might have settled badly in shipping. There's some chance the magnesium kit is bad, though, so I'd get a second opinion if that were easy. Other magnesium supplement might be less expensive, although I'd consider letting water changes raise the magnesium over time, or going the DIY route, since that level is rather low.

[oblio]

+1 to bertoni, jbambas, and downbeach, my guess is either your salt is actually low, for whatever reason, or your tester is off.
Mix up a small amount of fresh salt mix to your desired salinity. Check mag with your test kit, maybe get a second opinion to validate its one or the other.
Although its true batches of salt can vary, I have found IO to have more than enough mag to suffice.
My guess is its your tester, but that's just my guess.

[JJ70]

Thanks everyone. I will test a fresh batch of water tonight to see what I get.