Calcium Carbonate Precipitation after water changes

[Ilikecoral123]

So, about 2 Months ago I switched to Aquaforest Reef Salt. I mix it to 1.025 SG. I had no issues with the salt until the last two batches.


I purchase the 200g box of salt and mix my saltwater in batches. I mix it at a water temperature of 75°F. I use a 40-gallon Brute container with a return pump sitting at the bottom, blasting water toward the surface for circulation. I add salt using the provided cups, adding roughly 3–5 cups at a time until I reach the desired salinity.


My normal parameters are:

• Alk: 8.5–9.0 dKH
• Calcium: 400–450 ppm
• Magnesium: 1400–1450 ppm

I typically perform 20–30 gallon water changes.


I reached the end of the box of salt and didn’t think anything of it. After my second-to-last water change, the following morning I noticed the tank water had turned a white, milky color.


I tested my parameters:

• Alk: 9.8 dKH
• Calcium: 550 ppm
• Magnesium: 1550 ppm

My pH also dropped overnight from my normal peak of around 8.15 to 7.9. This was lower than my normal nighttime pH, but not dangerously low.


I triple-checked my results, and the numbers were accurate. All tests were performed using Hanna checkers. Salinity was measured using both a calibrated refractometer and compared against a Hanna salinity checker.


At this point, I didn’t suspect the salt mix. I assumed something had gone wrong with my dosing system. I immediately stopped dosing. (I also noticed some SPS experiencing rapid tissue necrosis during this time.)


Fast forward one week: I continued daily testing with no dosing.


Before my next water change, my parameters were:

• Alk: 9.0 dKH
• Calcium: 500 ppm
• Magnesium: 1500 ppm

Again, I had not dosed anything during this entire period.


I then mixed the remaining salt I had left from the box to 1.025 SG as usual. I performed a 20-gallon water change. About an hour later, I noticed the water became milky again.


I immediately tested the water:

• Alk: 9.8 dKH
• Calcium: 550 ppm
• Magnesium: 1550 ppm

I also noticed my pH dropped significantly after the water change.


At this point, I realized the issue was likely coming from the salt mix and not my dosing. There was nothing I could do except wait and monitor.


Unfortunately, I have had more SPS RTN during this process.


The next morning, before leaving for work, I checked parameters:

• Alk: 9.0 dKH
• Calcium: 500 ppm
• Magnesium: (I did not test before leaving; I will check when I get home.)

One thing I should mention: because I buy the 200g box of salt, I did not mix the dry salt inside the box before using it. I now realize this may have been a mistake, especially with a large quantity of salt where heavier components could potentially settle during shipping or storage. I am wondering if the final batches from the box may have had an uneven concentration of calcium, alkalinity, and magnesium.


I will add the batch number from the box once I have it available.


I also want to mention that I did not test the freshly mixed saltwater before adding it to the system. Looking back, this is something I should have done. Going forward, I will be testing every batch of mixed saltwater before adding it to any of my systems to verify the parameters and catch any issues before they affect the tank.


For those who use large boxes of salt, is there a recommended way to thoroughly mix the dry salt before use to prevent this from happening? Should I be breaking down the box and mixing the entire contents before storing it, or is there a better method?


Has anyone experienced something similar with Aquaforest Reef Salt? Could this be a bad batch, a mixing issue, or precipitation caused by an unevenly mixed dry salt blend?

 

[BryanM]

I think you hit the nail on the head to test the next batch of salt you mix.

AllI can say is that buy the 200 gallon bucket of Tropic Marin reef pro salt, and I've never bothered to do anything more than scoop salt from the top of the bucket.

But I'm not your ideal comparison here as I also don't do water changes... I make some very salty top off water every few months to bring my salinity back up to 35 ppt due to losses from testing, dipping, splashing.

 

[Randy Holmes-Farley Verified Community Expert]

So there is no solid salt left?

It may have settled during shipping to have more alk in the part you got to most recently.

It’s the elevated alk that will boost precipitation.

 

[Ilikecoral123]

So there is no solid salt left?

It may have settled during shipping to have more alk in the part you got to most recently.

It’s the elevated alk that will boost precipitation.

That's what I'm starting to think as well. Since this only occurred with the last two batches from the 200g box, I'm wondering if some of the components may have settled or separated during shipping/storage. I understand that this isn't something that has been definitively proven with aquarium salt mixes specifically, and I don't want to assume that a large box of salt will always become non homogeneous. However, segregation of mixed powders is a real possibility in other industries, so I think it's a plausible explanation worth considering.

The one thing I should have done (and will be doing going forward) is testing the freshly mixed saltwater before adding it to the tank. Since I didn't test it at the time, I can't say for certain whether the saltwater itself was already elevated or if something about the water change triggered the precipitation event.

My thought is that if the dry mix did become unevenly distributed, the final portions of the box could have contained a higher concentration of certain components, resulting in a more concentrated batch. That would explain why the issue only appeared at the end of the box and why the same remaining salt reproduced the issue twice.

Either way, this has definitely taught me to thoroughly mix the dry salt before using a large box and to test every batch of freshly mixed saltwater before it goes into the system. That way I can catch any issues before they impact the tank.

 

[Randy Holmes-Farley Verified Community Expert]

I talked to an Instant Ocean rep years ago who put a bucket of IO on his washing machine and left it there for a while. He did find that it settled, so it can happen.