Alkalinity Problem

[costa13]

I am wondering why my alkalinity gets so low sometimes. Even when my other parameters are good. I just tested my water and these are the results.
pH 8.1
Ammonia 0.1
Nitrate 0
Nitrite 0
Calcium 450
Alkalinty 5.6dkh
Specific Gravity 1.026
I just dissolved some sodium carbonate in a cup with ro/di water, and then added it to my tank to raise my alk. Is there any reason why my alkalinty gets so low?
Thanks in advance.

 

[reefnut]

What additives are you using?

 

[costa13]

I am not using any additives.

 

[reefnut]

What does it read now that you added the sodium carbonate?

 

[costa13]

hold on for just a second. I am testing right now

 

[costa13]

Okay
It is reading around 7.5dkh

 

[reefnut]

So you do not add anything?? how about water changes... what salt are you using?? Do you know the calcium and alkalinity readings of your newly mixed saltwater.

 

[costa13]

no I do not use any additives. I have been buying my saltwater from my LFS. They use IO salt and when they tested it everything was perfect. This weekend I am going to start making my own salt.

 

[costa13]

:help:

 

[acrylic51]

What do you have in your tank? At times certain items can be drawn quicker than before, meaning the consumption rates can change as your corals and such grow.....

 

[reefnut]

I'd test the water from the LFS. I agree with acrylic51 but that seems to me to be a dramatic difference... with the calcium being at 450ppm... unless there was an imbalance to begin with.