Why did my ALK drop?

[hoffmeyerz]

I did some maintenance and testing over the weekend and found a considerable drop in my Alkalinity. Tank is 75gal and is a year old. Parameters have been pretty stable at: (with Hanna checkers)
Sal - 1.025
PH - 8 to 8.1
NO3 - 15
PO4 - .09
ALK - was dkh-7 now it's 5.5
I don't have any corals yet so I was wasn't paying a lot of attention to my Alkalinity, I always wanted to raise it some but that was on the back burner.
I'm not sure first, why it dropped, and second, the best way of raising it. I've read so many ways to approach it that I'm not sure which way to go, or being that for now it's FOWLR if I should even do anything at all.
I appreciate any insight!

 

[unchaotic]

Not sure of the cause for the drop. I'd redo the test and maybe get another test kit to double check it.

Considering it's FOWLR you can probably raise it a little faster than normal but there's probably no need to rush it. ALK is super cheap to dose (RODI water and Baking Soda) so I'd do a little of that as well as your regular water changes (check the ALK on your freshly mixed water just to make sure that didn't cause the problem).

 

[hoffmeyerz]

Thanks. I did double check the test and got the same result. I do have some of the baking soda solution already made, I used it a long time ago to raise pH and alk. I had to keep adding it in every few days and I was worried about it precipitating back out so I focused on raising the PH and hoped ALK would follow. I added a co2 scrubber and that raised the PH, ALK only slightly bumped up but I decided to let that go since I had no corals.
I can start dosing it again, I'll start playing with dosing amounts and times. I'll also check my rodi and mixed sal water too.

 

[wakejumper22]

Have you checked your other values? Magnesium and calcium also have an effect on if alk will precipitate..

 

[hoffmeyerz]

No, I haven't been checking cal and mag. I didn't put an emphasis there since I wasn't putting corals in yet. I kept an eye on alk because I know that's tied to pH.
I was unaware of a relationship between mag and cal to alkalinity as well. I will look into that, thanks for mentioning it!