Fixing Alkalinity rise caused by Nitrate dosing

[NeveroddoreveN]

My Alkalinity has been gradually creeping up compared to my calcium and was stumped until I realized it was because of NaNO3 dosing. I dose about 4ppm nitrate per day for macroalgaes.

Would dosing calcium nitrate instead of sodium nitrate prevent that? Should I continue with the NaNO3 and just dose calcium chloride to compensate?

 

[Larry L]

I haven't heard that nitrate dosing causes alk to rise - are you doing 2-part and maybe what's actually happening is alk consumption is slowing down while you continue to dose the same amount of alk? If the increased nitrates are causing increased macroalgae growth, that might be depleting phosphates to the point your coral metabolism has slowed?

 

[NeveroddoreveN]

I haven't heard that nitrate dosing causes alk to rise - are you doing 2-part and maybe what's actually happening is alk consumption is slowing down while you continue to dose the same amount of alk? If the increased nitrates are causing increased macroalgae growth, that might be depleting phosphates to the point your coral metabolism has slowed?

I use kalkwasser

As i recently learned, when nitrate is made in the first step of the nitrogen cycle, alkalinity is depleted as you can see in reaction (1)

When nitrate is consumed by anoxic bacteria (2) or macroalgaes (3) it produces bicarbonate in a 1:1 ratio of the nitrate that was consumed.

I think that's why my alk was rising, by short circuiting to the second part of the N cycle where bicarbonate is produced.

My question is, would dosing calcium nitrate instead of sodium nitrate fix this imbalance?

 

[taricha]

My question is, would dosing calcium nitrate instead of sodium nitrate fix this imbalance?

The first level answer is no. You can see from Randy's equation, that the alk production is totally not involving any kind of calcium or calcification processes. Taking in the NO3 produces bicarbonate. regardless of whether that was KNO3 or NaNO3 or Ca(NO3)2

The second level answer is that your alk consumption must be very very low for nitrate uptake to outweigh Alk consumption by calcifying organisms.

Which brings us back to your question. I suppose if your Ca is absurdly low (you don't say what the Ca level is) then I suppose it's remotely possible that calcifying organisms in your tank are being limited by the Ca level, and adding Ca might help - but there's no reason to tie it to NO3 if that's so.
If your Ca is at plausible reef values, then no - it won't have an effect.

 

[Randy Holmes-Farley]

My Alkalinity has been gradually creeping up compared to my calcium and was stumped until I realized it was because of NaNO3 dosing. I dose about 4ppm nitrate per day for macroalgaes.

Would dosing calcium nitrate instead of sodium nitrate prevent that? Should I continue with the NaNO3 and just dose calcium chloride to compensate?

It would prevent alk from rising relative to calcium, but it does not prevent both from rising unless you dose less of other additives.

 

[Randy Holmes-Farley]

I haven't heard that nitrate dosing causes alk to rise -

It does.

 

[Larry L]

When nitrate is consumed by anoxic bacteria (2) or macroalgaes (3) it produces bicarbonate in a 1:1 ratio of the nitrate that was consumed.

Thanks, good to know!

 

[NeveroddoreveN]

I suppose if your Ca is absurdly low (you don't say what the Ca level is) then I suppose it's remotely possible that calcifying organisms in your tank are being limited by the Ca level, and adding Ca might help - but there's no reason to tie it to NO3 if that's so.
If your Ca is at plausible reef values, then no - it won't have an effect.

The calcium is a good point. When I said my alk was rising- I lied. My calcium was low. While not recommended, I had only checked alk (not Ca) for quite a long while, just dosing kalkwasser to keep it at a solid 11dkh and assuming that Ca was staying more or less in-step at around 450.

But my sps totally stopped growing and I checked my Ca and it was 290-300! I was scratching my head pretty hard because none of the other reasons for huge Ca consumption (compared to alk) fit my system or could only account for a few percentage points of difference.

Then I learned that nitrate dosing increases alk and it all clicked: I had started (heavily) dosing NaNO3 a few months ago. By only testing and maintaining alk, my calcium was gradually decreasing to such a point that my corals couldn't really calcify anymore.

Lessons:
(1) Always test Ca too (duh)

(2) Short-circuiting the nitrogen cycle by nitrate dosing really really can change ionic ratios of Ca/Alk, especially if you over-do it.

 

[NeveroddoreveN]

It would prevent alk from rising relative to calcium, but it does not prevent both from rising unless you dose less of other additives.

Thanks!

In the future I'm going to always recommend dosing calcium nitrate instead of sodium nitrate. I think most folks use some balanced additive(s) for Ca/alk and it's usually less problematic/confusing to sightly increase them both in lock step than just sightly throw one off.