Denitrification and alkalinity

[MrGisonni]

I'm a fairly advanced hobbyist with a couple of decades of experience yet every time you think you have a handle on things there is more to learn. My latest setup is a 30 gallon mixed reef. The tank has been set up for 9 months now and has had its ups and downs. I've been using All for Reef and doing weekly 10% water changes with natural sea water since day one. It took me a few months to get things stable with alkalinity and calcium but seem to have found the sweet spot. Even with water changes a few months ago my nitrates crept up to 15 parts per million. I used Brightwell Aquatics export nitrate cubes in a plastic container in my sump with great success. Nitrates are now under 5 PPM but what I have noticed is my alkalinity has been increasing. Only up to 9.7 but enough for some of my sps to burn at the tips and others to clearly stress out. The question is.... Does the denitrification process add significantly to alkalinity? Should I reduce my AFReef or remove some cubes?

 

[Auquanut]

Good question. Following. I've always associated increased Alk with less than happy corals, but...

 

[Dennis Cartier]

No, the denitrification will only return the alkalinity that was used when the nitrate was created. So it ends up being a wash. Except in cases where a reefer doses nitrate directly. Then, that can raise alkalinity as it was not lowered during the creation of the nitrate, as no nitrate was created in the tank, and an increase in alkalinity results after the nitrate gets consumed, or in this case denitrified.

 

[arking_mark]

Per @Randy Holmes-Farley :

A drop of 50 ppm in nitrate (except by water chantge) boosts alk by exactly 2.3 dKH

So dropping nitrates 10ppm via denitrification will result in roughly a 0.5dKH increase.

 

[MrGisonni]

Which is in the ballpark of what I saw

 

[GARRIGA]

Why I strive to ensure I have a working denitrifying filter. No sense throwing that out with water changes.

 

[Randy Holmes-Farley Verified Community Expert]

Bear in mind the effect only comes into play if nitrate is rising, falling, or being dosed.

Without dosing, a steady nitrate level (any level) indicates no ongoing net effect on alkalinity.