What’s absorbing my alk

[Glenng78]

I have very few corals in my 140 gal display, and they are all zoanthids at that. I started my tank with dry liferock from caribsea, and also dry sand. I also have a couple bricks of marinepure in my sump. The tank has been up and running since around February 2019. My question is this. I just got a trident and I have noticed that my alk falls to around 8dkh every day from around 9. So it is a 1 dkh drop every day. I dose it back up every night, only for it to drop again. Does anyone have any insight on what might be causing this? I don’t think some zoanthids would pull that much alkalinity. Is it possible my rock is absorbing it ?
Calcium - 420
Mag- 1380
Ph-8.09

 

[SteveEreef]

Do you have a lot of corralline algae?

 

[Glenng78]

No coralline

 

[Goodair]

Any inverts or calcified macro algae?

 

[Glenng78]

A couple pistol shrimp. And very few snails. I have a powder blue and yellow tang. Pair of clowns. Bengai cardinal, and watchman goby.

 

[Glenng78]

No macro algae

 

[W1ngz]

Is calcium also stable? Are you dosing anything else?

 

[IXP7633]

GFO?

 

[Glenng78]

No GFO ,
I dose b ionic 2 part - but the calcium only once a week really- the calcium is pretty stable.
Is it normal for my alk to drop like that overnight without serious coral consumption ?
Or is it possible my rock/ sand absorbs it ?

 

[W1ngz]

The rock and sand don't absorb alkalinity. It may be precipitating there though. Is the sand hard or clumping up? It may be precipitating.

If you stop dosing the alk for a few days, what happens? Does it stabilize? With no stony corals there's really no reason to keep your alk above 8.

 

[Glenng78]

No clumping . I haven’t really stopped for a few days. I’m ok with an 8dkh, but should it drop from 9 to 8 with nothing using it ?

 

[W1ngz]

Yes it's possible, dry sand and dry rock can sometimes cause precipitation when the levels are high in a new tank. The rock isn't absorbing the alk, it's actually losing small amounts of calcium and causing the alk to be pulled out of the water where it's likely being deposited on the sand in small amounts.

Let it fall off for a few days and see what happens, but keep it above 7.

 

[Glenng78]

Please explain what precipitation is ? I thought that meant it would leak INTO my tank

 

[W1ngz]

Precipitation is when dissolved compounds in the water solidify and fall out of the water to the bottom.

When you have high concentrations of calcium and sodium bicarbonate (typical alkalinity buffer) in the same place, they combine to make calcium carbonate. This is the same stuff that corals produce through biology for their skeleton, but in abiotic (without biology) precipitation it's just a powder or dust that likely is getting lost in the sand.

The high calcium may be only happening in close proximity to the rock, and getting bound by the excess alkalinity before it ends up free in the water, which is why you aren't seeing it in your tests.

 

[Glenng78]

Well that was the most complete explanation I have ever heard. What course of action do you recommend

 

[W1ngz]

Let it fall off for a few days and see what happens, but keep it above 7.

 

[Glenng78]

I’m down to 7.7 as of noon today March 7. That’s down from 8.89 at around 1am on the morning of March 5.
calcium -398
Mag - 1381
Ph- 8.02

 

[Randy Holmes-Farley]

I’d stop trying to keep it at 9 dKH. 7-8 dKH is fine, and the amount needed daily to maintain that will be lower.

Are you measuring nitrate? Rising nitrate will also consume alkalinity.