Alkalinity problem

[Dub]

I have Red Sea reefer 170 that including the sump has 43 gal. The tank has been running 5 months and all conditions are stable, ca 420, alk 6.5, ph 7.9, nitrate 2.5 and phosphate 3.0. I have been doing water changes as follows; Using Frits reef pro salt 11% change every 2-3 weeks then using Red Sea Coral Pro salt 25% change for last 2 changes (3 weeks between changes). S.P. Set to 1.026. I have 4 fish , 4 rock flower nems and clean up crew.

My problem is the Alk. I have adjusted it with Fluval Sea alkalinity to 9.3 when I started using Red Sea salt 4 weeks ago only to find it had dropped to 6.3 last week when I adjusted it to 8.5 only to find it back to 7.0 today. What is causing the drop? I would like to try sps but can't get alk high enough. I plan on another water change tomorrow.

Thanks for any suggestions

 

[Randy Holmes-Farley Verified Community Expert]

Alkalinity always drops in reef tanks as organisms use it and calcium to make skeletons and shells and such. You should expect to dose alkalinity frequently, maybe daily or even multiple times per day.

Water changes can’t keep up with anything but the lowest alk demand.

 

[SeaDweller]

Btw... you don't really need alk higher than 7.0-7.5 dkh to keep SPS, but you do have to maintain a stable number, whether it be 6.5 7.0 7.3 7.5 7.8, etc. Personally, I wouldn't shoot for an alk higher than 8.0, but that's just me. You'll see people keeping tanks at a wide range of alk, but there are other factors that play into it too.