Alk keeps testing low 5.7 to 6.3

[JonasRoman]

I have had the same issue since the beginning original post is here : https://www.reef2reef.com/threads/new-tank-cycle-finished-—-trying-to-figure-out-what’s-eating-my-alk.1154168/

Tank is a Waterbox 110.4 with live sand, base rock.

Equipment: Protein skimmer with CO2 scrubber attached, UV.

Fish: Sailfin, Foxface, 2 clowns, 3 Cardinals, Goby

Corals: RBTA, Branching Hammer, 3 different types ZOA’s (not happy), Blastos, 4 Acros and encrusting monti Jedi Mind trick (looks amazing)

Salt used is Red Sea Pro. (I mix it for an hour when doing water changes)

Sump light runs at night with chaeto.

My pH peaks around 8.2 during the day and drops to 7.8 by morning

Salinity Refractometer: 1.026

Test results with Salifert Kit less than a month old:
• 09/11 6.1 DKH Cal 360 Mag 1155

• 09/16 5.7 DKH Cal 345 Mag 1155

Thought I had a salinity problem, so I had my local shop calibrate my refractometer. My fluid is 6 months old and shows 1.030 vice the 1.026 it should show.

So what could be using up all my alkalinity this fast? And is there anything else I can do to stabilize everything?

Reason could be precipitation of newly mixed salt due to supersaturated solution. A few things you can do wrong when particularly mixing red sea pro:
1) never put in salt before 100% of the RO water volume is ready.
2) do not put in all salt at same time. Do it in portions
3) do not heat up water to more than 20 degrees.
4) wait absolutely no more than one hour

Otherwise precipitation occurs and Ca and alk depletes as CaCO3 is created. And once that happens you can not bring it back. The salt is "destroyed" forever.
Red sea pro is especially prone to this as its supersaturated.
Its very unlikely its a true consumption once it reaches the tank because your Ca was low too meaning such consumption of animals in such short time is absolutely impossible. So again, I vote for precipitation when you mix the salt. Follow the rules above and that will not happen.

Jonas

 

[CaptClay]

UPDATE: In case anyone cares, I wanted to share what I’ve figured out. For a while I suspected the issue was the Red Sea Pro salt. I bought new test kits, had multiple people calibrate my refractometer, and tried a handful of other adjustments. Eventually I switched back to Instant Ocean two weeks ago (which I used for decades with great results).

After just two water changes—a 15‑gallon and a 20‑gallon—my tank parameters have improved significantly:

• Alk: 8.0
• Cal: 375
• Mag: 1215
• NO3: 5
• PO4: 0

At this point, I’m convinced Red Sea Pro is just too finicky for a hobbyist setup. I’ve been using it since March of ’26 and had nothing but problems. Instant Ocean seems to be giving me much more stable results.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks for the update. :)

 

[JonasRoman]

UPDATE: In case anyone cares, I wanted to share what I’ve figured out. For a while I suspected the issue was the Red Sea Pro salt. I bought new test kits, had multiple people calibrate my refractometer, and tried a handful of other adjustments. Eventually I switched back to Instant Ocean two weeks ago (which I used for decades with great results).

After just two water changes—a 15‑gallon and a 20‑gallon—my tank parameters have improved significantly:

• Alk: 8.0
• Cal: 375
• Mag: 1215
• NO3: 5
• PO4: 0

At this point, I’m convinced Red Sea Pro is just too finicky for a hobbyist setup. I’ve been using it since March of ’26 and had nothing but problems. Instant Ocean seems to be giving me much more stable results.

I am still curious how you exactly do when mix red sea pro? I have used this salt for decades with not a single issue.
Its a few easy things to care about to avoid precipitation(but so easy so I honestly do not think about it actively)
1) only pour salt into RO (always ro) when all ro water are ready. So you don't get too high salinity
2) do not warm up water to more than 20- 23 degree ish and ok to do it after the salt has been added
3) when salt added use a powerhead and mix but approx not longer than approx 1 hour.(or even shorter)
If you do this I am confused you got precipitation.
Anyway, there is other good salt so go for something else if you find it struggling