Mostly Alkalinity Being Consumed.

[Paulsz]

HI all,

I have a 35 cube + 20 high as a sump (filled halfway). Started it back in December and it's now got one one firefish, a CUC and 4 corals:

- GSP (roughly 1.5"x1.5")
- sympodium (one frag plug)
- 1 orange hair mushroom (1" across)
- purple MontI 2 pieces, about 2" long, 1" outwards)

I'm using Red Sea Coral Pro salt and have the following parameters:
- temp 77F
- pH 7.9 to 8.1
- Cal 430-435
- Mg 1400
- Alk 8.1 dKh at the moment

I do about 12% water change (5 gals) every two weeks. I used to do it weekly but the bioload is fairly small

One thing I've noticed is that when I test every week, I see that my Cal and Mg stay the same (i.e. the water change is enough to supplement what was lost). But my Alk drops about .5-.7 dKh. It was 9.5 two weeks ago, dropped to 8.7 last week, then 8.1 this week. Is this normal considering I only have e a couple small pieces of Monti? The MontI is growing, but growth is fairly slow. And why don't I see the other elements drop?

 

[SecretiveFish]

In my experience, yes this is normal. My husband would regularly have to fill the alkalinity doser much, much more frequently than the calcium. I suspect it has something to do with CO2 being absorbed by saltwater and binding with alkalinity components... We go through a lot of alkalinity solution to maintain PH.

 

[Nart]

That seems normal. small variables in testing results are often just the +/- accuracy of the test and not including the slightest human error. The biggest variable is the exact amount of water drawn each time. Like, are there any small bubbles in the syringe when the sample water was collected? Did any drop of water drip out during the drawing of the syringe? and sterile cleaning of the test tube. Just things like that can skew your test results by the slightest.

Also, water changes can affect it too. Like, if you mixed your salt at 1.026 this week, but next week you mixed it at 1.0255 that's also a drop in Alk, so when you do the 12% water change you're lowering your Alk a bit as well. You could always take it a step further and measure your Alk from the fresh batch of saltwater mix to make sure it is within range of your tank as well.