Drop in CA/MG - Is sudden Coralline algae growth responsible?

[dasickle]

Hi all,

My CA is usually 425 - 450 and my MG is around 1250 - 1300. I do my 1g water changes every 2 - 3 days on a 3g pico. I also dose 2 ml (pre-mixed) of Brightwell Aquatics Magnesium every other day. Other levels are good as well:

Ammonia - 0
NO2 - 0
NO3 - under 5
SG - 1.024
PH - 8.3

Did last water change 2 days go and decided to run my tests. CA came up at 345/Alk 8.8 and my MG 1050.

What could cause this swing in just two days? My numbers are alway very predictable in the tank and everyone seems very happy.

I did notice the first sign on Coralline algae on my LR which didn't have any on it before. Could that be the cause?

OR

I recently bought a 3 inch Tridacna gigas and I hear they can absorb quite a bit of calcium. Could that be the cause?

Maybe both?

Everything

 

[bertoni]

Clams and coralline both can consume a lot of calcium and alkalinity. That drop in magnesium looks more like a testing problem or a water change with a bad batch of salt, though. I'd double-check that.