Why is Calcium precipitating?

[Icey MD]

I think this white material is CaCO3. its forming big chunks as well as a white dust (more visible in the blue light). The tank is 3 months old and ive noticed this happening for about 1 week. Started with dry rock). Ca 400 (down from 420 last week). KH constant at 143. pH 8.1 . (I need to pick up a Mg test kit, but I have no reason to suspect Mg should be low).

Any ideas why this is happening, and what should I do about it? Thanks in advance.

 

[saltyfilmfolks]

It’s not what cal precipitation looks like generally.

Is it in piles that blow off?

 

[Icey MD]

some of it blows off sort of like flakes, but it leaves a white base that doesn't scrape off (i havent tried a blade)

 

[Damien Buckley]

Not enough mag?

 

[Randy Holmes-Farley Verified Community Expert]

I agree that it may not be calcium carbonate precipitation.

When you have a precipitation issue, the things to look at are:

1. High alk and pH (these are the biggest drivers by far; including local events such as where and when you add high pH alk additives)
2. Low magnesium, phosphate, and organics
3. New, fresh calcium carbonate surfaces that act as seed crystals

 

[Icey MD]

Ok thanks. If it's not calcium precipitation, any ideas what else it could be? When I initially posted this in the algae section, they seemed to think it was a precipitate, rather than an algae.

 

[Randy Holmes-Farley Verified Community Expert]

It's hard to tell from the photos, but precipitation would not usually cause an apparent white area on a largely unimpacted rock surface. Much of he time, it goes unseen as hardening of the sand and precipitation on warm objects like pump impellers and heaters.

I'd vote for an organism of some sort (not algae, which is never white in a lit location). Bacteria? Sponge? etc.