Sulfate Off The Chart

[Euphylliaphyle]

My ICP-MS came back with sulfate at 3,593 mg/l. I have seen some old posts where high sulfate was not a concern, or the effect was not well understood. Has this changed since c.a. 2020?
At some point this has to be a concern. I mean, our reefs cannot survive at 1,000,000 ppm, to take it to a logical extreme, right?
Part 2: Where would this come from? I rarely (1x/week?) feed pellets, just few as a distraction to fish & inverts while I target feed corals. I feed frozen otherwise.
Part 3: How do I get this down, if needed? I do water changes, 16% every 10 days.

 

[Randy Holmes-Farley Verified Community Expert]

Water changes are the only way to lower elevated sulfate.

Are you using a lot of Epsom salt for magnesium?
It is quite high.

 

[Euphylliaphyle]

Water changes are the only way to lower elevated sulfate.

Are you using a lot of Epsom salt for magnesium?
It is quite high.

I dosed Mg from Epsom salt from 1/4/26 until 1/8/26, 25 ml twice/day for a total of 240 ml (in a 32 gallon system). I then dosed alk from baked baking soda until 1/28/26, then switched to AFR once Alk was good. I have used AFR and have done 6 water changes of 16% since then until the ICP sample date. I dosed a small amount of Epsom salt solution once since then to get Mg back in line.

Edit: By my reckoning, even if the cumulative Mg dosing was 100% sulfate (obviously it is not) that would only be ~0.3 liters in a 121 liter system, or 2,479 ppm. Cumulative, no export.

 

[Randy Holmes-Farley Verified Community Expert]

Do you recall what ppm boost that was? Epsom salt adds 4 ppm of sulfate for each 1 ppm of magnesium, so you would need a large boost to get sulfate that high.

 

[Euphylliaphyle]

Do you recall what ppm boost that was? Epsom salt adds 4 ppm of sulfate for each 1 ppm of magnesium, so you would need a large boost to get sulfate that high.

From what I can reconstruct, I wanted to go from 1,207 to 1,300 over 4 days' time, adding 93 ppm Mg (and therefor 372 ppm sulfate?) If I'm interpreting Oceamo's "ideal value" column correctly, that is NSW value? If so, at ideal = 2,569, I'm looking for a delta of 1,024. The Mg dosing can account for 1/3, but does sulfate have that kind of staying power through 6 water changes?

 

[Randy Holmes-Farley Verified Community Expert]

35 ppt natural seawater magnesium is 1280 ppm. :)

 

[Randy Holmes-Farley Verified Community Expert]

Sulfate is removed by water change as the weighted average of the value in the new and old water. There is no special carryover as there can be for phosphate.

 

[Euphylliaphyle]

35 ppt natural seawater magnesium is 1280 ppm. :)

You go for nsw Mg in your system? I was aiming to run rich on purpose.

 

[Euphylliaphyle]

Sulfate is removed by water change as the weighted average of the value in the new and old water. There is no special carryover as there can be for phosphate.

Okay, thanks. That's what I imagined, but just guessing my way through this.

 

[Randy Holmes-Farley Verified Community Expert]

You go for nsw Mg in your system? I was aiming to run rich on purpose.

It’s fine to run a bit higher. Mine was 1417 ppm from the normal
IO and AFR that I use and salinity a bit higher than normal at 36.2 ppt when tested by icp.