Salinity plummeting, and low dKH. Help!

robgreaves · 01/19/2017, 02:21 PM

Hi, I'm a new member in Devon, UK.

I have a 100 litre marine tank, with approx 11kg of matured live rock in it.

I set the tank up to an SG of 1.025, using Kent Reef Crystals, with RO water.

From the initial mix, I arrived at my target 1.025, but alkalinity was low at dKH 6.8.

Adding a little Red Sea Nitrobac to help the bacterial cycling on its way, and a little Red Sea Reef Foundation B buffer to raise the alkalinity, the alkalinity did slowly come up, but the pH didn't.

I have good flow from 2x wavemakers, and I'm running a Tunze in-tank 9001 skimmer.

However, over the course of the past week or so, my salinity has been dropping. It's now at 1.018. Alkalinity is up around 9.5 or so.

Other parameters measured this evening include:

Calcium: between 490 and 520ppm
Magnesium: 1100ppm
Nitrate: 50ppm

I am also seeing the odd white 'snowflakes' appearing in my tank, which I assume is precipitated Calcium given the high(ish) alkalinity.

SO!... Any ideas why my salinity is plummetting? What's the best course of action? Add salt solution slowly? I can't see it would just 'come good' on its own.

I'm measuring using a Red Sea refractometer, calibrated at 25C using RO water to zero it.

Any help or advice welcome.

Thanks - Rob

nereefpat · 01/19/2017, 03:51 PM

The snowflakes could be precipitation of Ca, as you stated, but the problem would be the low Mg. Mg should be a couple hundred ppm higher, especially with that high Ca. I think the alk is in the safe range, but the high Ca and low Mg is likely to give you trouble.

As for the salinity dropping; this is only possible if you have a leak in the system, and the leaked saltwater is replaced with fresh. OR your refractometer is off. I would like to see you use a 35ppt standard, rather than the RO water.

Tripod1404 · 01/19/2017, 03:57 PM

Salt cannot evaporate, so there is now way for significant amounts of salt to leave the system on its own. Therefore, salinity would not drop on its own unless saltwater is somehow replaced by freshwater. I suspect there i something wrong with the measurement.

Sk8r · 01/19/2017, 04:02 PM

Something is highly 'off' here. Suggest a reading of the SETTING UP sticky in New To the Hobby, which covers a broad number of topics.

YOur SG situation is just a can't-happen without either an input of fresh water or a real problem reading your instrument. If you are using a swing-arm salinity measure, those are notoriously inaccurate. You have to bump them repeatedly to get all bubbles off the arm. Get a refractometer and shoot for 1.024 salinity steady. Draw a 'fill-line' on the glass to be sure of your proper water level, and keep it there.

Dan_P · 01/19/2017, 06:34 PM

Not all salts or their anions contribute equally to specific gravity. I don't recall the magnitudes, you'll have to look this up, but the calcium precipitation could be giving the mysterious salinity drop.

Tripod1404 · 01/19/2017, 10:01 PM

Quote:

Originally Posted by Dan_P

Not all salts or their anions contribute equally to specific gravity. I don't recall the magnitudes, you'll have to look this up, but the calcium precipitation could be giving the mysterious salinity drop.

Saltwater contains ~19000 ppm chloride ion, ~10500 ppm sodium ion and ~450ppm calcium ion. If we make a broad assumption that only these 3 ions are present, Ca would only make 1.5% of total ions (with all those other ions it would be even less). So large sums of Ca needs to precipitate for it to have an effect on total salinity.

bertoni · 01/20/2017, 01:12 AM

SG will drop only if fresh water is added to the system, barring very extreme situations. How are you measuring SG? Hydrometers and swing-arms are prone to measurement issues.

karimwassef · 01/20/2017, 04:14 AM

SG will drop if fresh water is added (is your ATO or Alk doser running wild) and/or a lot of salt is removed (do you have a leak?)

If you SG is dropping & Alk is rising, I would looking at your Alk doser running amok (if you have one).

robgreaves · 01/22/2017, 08:08 AM

Problem solved... turns out my (second hand) refractometer was measuing all over the place.

I splashed out and spent $175 on a Milwaukee digital seawater refractometer. Turns out that when my optical Red Sea refractometer was reading 1.025, in reality, it was 1.014.

Now I've added more salt, and brought the salinity correct, the pH has come up too - as you'd assume.

All is good in the tank, and some Zoanthids have been added yesterday.

Thanks! Rob.

bertoni · 01/22/2017, 09:01 PM

Okay, that sounds like good progress! I'm sorry to hear that your refractometer was wrong, but at least you're back on track.

01/19/2017, 02:21 PM	#1
robgreaves Registered Member Join Date: Jan 2017 Location: Devon, UK Posts: 2	Salinity plummeting, and low dKH. Help! Hi, I'm a new member in Devon, UK. I have a 100 litre marine tank, with approx 11kg of matured live rock in it. I set the tank up to an SG of 1.025, using Kent Reef Crystals, with RO water. From the initial mix, I arrived at my target 1.025, but alkalinity was low at dKH 6.8. Adding a little Red Sea Nitrobac to help the bacterial cycling on its way, and a little Red Sea Reef Foundation B buffer to raise the alkalinity, the alkalinity did slowly come up, but the pH didn't. I have good flow from 2x wavemakers, and I'm running a Tunze in-tank 9001 skimmer. However, over the course of the past week or so, my salinity has been dropping. It's now at 1.018. Alkalinity is up around 9.5 or so. Other parameters measured this evening include: Calcium: between 490 and 520ppm Magnesium: 1100ppm Nitrate: 50ppm I am also seeing the odd white 'snowflakes' appearing in my tank, which I assume is precipitated Calcium given the high(ish) alkalinity. SO!... Any ideas why my salinity is plummetting? What's the best course of action? Add salt solution slowly? I can't see it would just 'come good' on its own. I'm measuring using a Red Sea refractometer, calibrated at 25C using RO water to zero it. Any help or advice welcome. Thanks - Rob

01/19/2017, 03:51 PM	#2
nereefpat Registered Member Join Date: Apr 2016 Location: Central Nebraska Posts: 3,190	The snowflakes could be precipitation of Ca, as you stated, but the problem would be the low Mg. Mg should be a couple hundred ppm higher, especially with that high Ca. I think the alk is in the safe range, but the high Ca and low Mg is likely to give you trouble. As for the salinity dropping; this is only possible if you have a leak in the system, and the leaked saltwater is replaced with fresh. OR your refractometer is off. I would like to see you use a 35ppt standard, rather than the RO water. __________________ Pat Current Tank Info: 125 in-wall , 40b sump. 6 bulb T5. ASM G2 skimmer. LPS and leathers

01/19/2017, 04:02 PM	#4
Sk8r RC Mod Join Date: Feb 2006 Location: Spokane WA Posts: 34,628 Blog Entries: 55	Something is highly 'off' here. Suggest a reading of the SETTING UP sticky in New To the Hobby, which covers a broad number of topics. YOur SG situation is just a can't-happen without either an input of fresh water or a real problem reading your instrument. If you are using a swing-arm salinity measure, those are notoriously inaccurate. You have to bump them repeatedly to get all bubbles off the arm. Get a refractometer and shoot for 1.024 salinity steady. Draw a 'fill-line' on the glass to be sure of your proper water level, and keep it there. __________________ Sk8r Salinity 1.024-6; alkalinity 8.3-9.3 on KH scale; calcium 420; magnesium 1300, temp 78-80, nitrate .2. Ammonia 0. No filters: lps tank. Alk and cal won't rise if mg is low. Current Tank Info: 105g AquaVim wedge, yellow tang, sailfin blenny,royal gramma, ocellaris clown pair, yellow watchman, 100 microceriths, 25 tiny hermits, a 4" conch, 1" nassarius, recovering from 2 year hiatus with daily water change of 10%.

01/20/2017, 01:12 AM	#7
bertoni RC Mod Join Date: Mar 2002 Location: Mountain View, CA, USA Posts: 88,616	SG will drop only if fresh water is added to the system, barring very extreme situations. How are you measuring SG? Hydrometers and swing-arms are prone to measurement issues. __________________ Jonathan Bertoni

01/20/2017, 04:14 AM	#8
karimwassef Registered Member Join Date: May 2004 Location: Dallas, TX Posts: 11,033	SG will drop if fresh water is added (is your ATO or Alk doser running wild) and/or a lot of salt is removed (do you have a leak?) If you SG is dropping & Alk is rising, I would looking at your Alk doser running amok (if you have one). __________________ Failure isn't an option It's a requirement. 660g 380inwall+280smp/surge S/L/Soft/Maxima/RBTA/Clown/Chromis/Anthias/Tang/Mandarin/Jawfish/Goby/Wrasse/D'back. DIY 12' Skimmer ActuatedSurge ConcreteScape

01/19/2017, 03:57 PM	#3
Tripod1404 Registered Member Join Date: Mar 2016 Posts: 1,821	Salt cannot evaporate, so there is now way for significant amounts of salt to leave the system on its own. Therefore, salinity would not drop on its own unless saltwater is somehow replaced by freshwater. I suspect there i something wrong with the measurement.

01/19/2017, 06:34 PM	#5
Dan_P Registered Member Join Date: Aug 2014 Posts: 1,432	Not all salts or their anions contribute equally to specific gravity. I don't recall the magnitudes, you'll have to look this up, but the calcium precipitation could be giving the mysterious salinity drop.

01/22/2017, 08:08 AM	#9
robgreaves Registered Member Join Date: Jan 2017 Location: Devon, UK Posts: 2	Problem solved... turns out my (second hand) refractometer was measuing all over the place. I splashed out and spent $175 on a Milwaukee digital seawater refractometer. Turns out that when my optical Red Sea refractometer was reading 1.025, in reality, it was 1.014. Now I've added more salt, and brought the salinity correct, the pH has come up too - as you'd assume. All is good in the tank, and some Zoanthids have been added yesterday. Thanks! Rob.

01/22/2017, 09:01 PM	#10
bertoni RC Mod Join Date: Mar 2002 Location: Mountain View, CA, USA Posts: 88,616	Okay, that sounds like good progress! I'm sorry to hear that your refractometer was wrong, but at least you're back on track. __________________ Jonathan Bertoni