What is causing Alkalinity to decline?

[Orion]

Hi there..

 

Tank  (57 rimless w/20gal sump) is almost 5 months old. Has no corals in it, and just one small BTA. I use Reef Crystals for salt mix. It continuously tests above 10.0dkh after 24hrs of mixing. 

My tank after a 10-15% weekly water change tests around 8.1dkh, and declines every day after. It’s been 4 days since I last did a water change and alkalinity is currently now at 7.56dkh (I tested twice to be sure). 

What is causing the alkalinity to precipitate?

 

I did dose two bottles of Coralline Algae (Pink & Purple) in a bottle by ARC Reef a month ago. I don’t see any growth yet.. could this be causing the alk to decline?

Fish:
Melanurus wrasse

chestnut blenny

lightning maroon clown

tomini tang

Current parameters:

 

temp 78

Salinity 1.026

pH 8.1-8.3 (Apex)

Alk 7.56 (Hanna)

Calcium 500 (Red Sea)

Mag 1400 (Red Sea)

Nitrate 5ppm (Salifert)

Phosphate 0.04 (Hanna)

 

 

[WheresTheReef]

At this point I wouldn’t worry about it based on your tank. Keep up with water changes and you will be ok. Once you start keeping calcifying corals that consume alk/ca then you can dose if you have a hard time keeping up with the demand. Could be the coralline algae growing. You may also have a little precipitation, but not a big issue if you aren’t seeing build up on pumps, heaters etc.

 

The production of nitrates also consumes alkalinity. I think it is around 2.3dkh for every 50ppm of nitrates produced. Anaerobic bacteria can take a long time to establish which converts nitrates into nitrogen. Luckily this returns the lost alkalinity back into the tank. I saw the same thing in my tank until it hit about a year. Things stabilized by then and I was able to cut my dosing down. The science behind this can be found here http://reefkeeping.com/issues/2004-12/rhf/index.php

 

 

[Origami]

As mentioned, an incomplete nitrogen cycle leading to elevated nitrate that is then managed with a water change will lead to an alkalinity decline. 

 

I suspect that you have some broad calcification going on, possibly as coralline algae that you haven't yet begun to notice. Another possibility is abiotic precipitation.