Why is my Alk dropping independently of other parameters?

[GhostInTheShell]

The alkalinity in my tank seems to be dropping steadily (about .1 dKH/day) independent of the other parameters. All parameters are tested daily because my humans are new to reefing and want to understand how everything works together.

The other parameters remain relatively steady at:
Temp: 78.1F
Salinity: 1.025
Ammonia: 0 ppm
Nitrites: 0 ppm
Nitrates: 0 ppm
Phosphates: .01 ppm
pH: 8.1
Calcium: 440 ppm
Magnesium: 1430 (+/- 15) ppm

10% Water changes bounce the alkalinity back to 7/7.1, but then it drops all over again.

My tank is about a month old. It was seeded with Arag-Alive Sand, small amount of live rock, and IO bottle bac, and cycled very quickly. There is a small microalgae bloom (looks like diatoms, cyano, or dinos…not sure). There is also coralline on several of the live rock pieces.

Tank Specs:
Tank: Fluval Evo 13.5g with stock pump and light
Filter: Filter floss, ChemiPure Blue, SeaChem Matrix
Livestock: Me (baby hermit crab)

Can anyone help my humans understand this chemistry strangeness and advise them on what to do about it?
-Ghost

 

[Randy Holmes-Farley]

At a drop of 0.1 dKh per day (assuming that is real), even if it is all used to form calcium carbonate, the expected drop to calcium is only about 0.7 ppm per day and magnesium is less than 0.1 ppm per day. it would take a few weeks to barely detect that calcium drop with a kit, and water changes in the meanwhile will swamp out any detectable change. You'll never detect a magnesium drop.

 

[Randy Holmes-Farley]

Most reef tanks use 0.5 to 3 dKH per day, so it's expected to see alk drop every day.

 

[GhostInTheShell]

Thank you so much! Should they do anything about it (Soda ash?) in between water changes, or just let it drop throughout the week?

 

[Randy Holmes-Farley]

Thank you so much! Should they do anything about it (Soda ash?) in between water changes, or just let it drop throughout the week?

At 0.1 dKH per day, I would only dose once every few days. Say, 0.3 dKH every third day, and see what that does over time.

 

[GhostInTheShell]

Great! Thank you so much for your expertise. When I am not busy sand-sifting and snail-murdering, I have been working my way through your water chemistry articles.

 

[Randy Holmes-Farley]

You're welcome and thanks!

Happy reefing.