Why does my alkalinity keep dropping?

[StrombusEyes]

New reefer here. Had a mixed tank set up 3 months ago. It’s a 100L all in one ocean free Marine. Up until a month ago my parameters were stable with alkalinity at 8.0. For the past few weeks I can’t keep the alkalinity above 6. The only changes I’ve made in this period is feeding more often (frozen food 3x day). I have noticed an increase in coralline algae but really small amounts on the glass. The phosphate has dropped in this period but calcium and magnesium remain in the normal range. My corals haven’t really grown, sps was only added a month ago and has not grown so I don’t think it’s consuming anything. The nitrate has also dropped to 0 in this period.
I do once weekly 15% water changes
I tried adding alkalinity solution which increased it temporarily to 7.2 but it dropped to 6 the following day.
Can anyone shed light on what is using the alkalinity?
Also if I need to supplement- wouldn’t daily dosing keep swinging the alk around?

Have attached photo of my tank and spreadsheet of my readings

 

[Saltyreef]

New reefer here. Had a mixed tank set up 3 months ago. It’s a 100L all in one ocean free Marine. Up until a month ago my parameters were stable with alkalinity at 8.0. For the past few weeks I can’t keep the alkalinity above 6. The only changes I’ve made in this period is feeding more often (frozen food 3x day). I have noticed an increase in coralline algae but really small amounts on the glass. The phosphate has dropped in this period but calcium and magnesium remain in the normal range. My corals haven’t really grown, sps was only added a month ago and has not grown so I don’t think it’s consuming anything. The nitrate has also dropped to 0 in this period.
I do once weekly 15% water changes
I tried adding alkalinity solution which increased it temporarily to 7.2 but it dropped to 6 the following day.
Can anyone shed light on what is using the alkalinity?
Also if I need to supplement- wouldn’t daily dosing keep swinging the alk around?

Have attached photo of my tank and spreadsheet of my readings

You may have an alk demand due to binding or consumption. Best bet is to measure the daily drop with a good test kit and start dosing. You can put your doser on a timer to dose small amounts every half hour to negate any swings.
Some all in one dosers have the timer built in. I use BRS dosers on kasa power strips and use the timer function through the app.

 

[ReneReef]

Based on these measurements, your tank is consuming 0.3 KH per week. That is very little and in line with your current stocking.

So, nothing weird going on.
And a KH of 6.2 is not bad by itself. The ocean KH is about 6.5.

It is clear the water changes alone are not sufficient anymore.
At this point I would start dosing alkalinity weekly to add 0.3 KH using sodium bicarbonate.
To increase KH back to around the 8 mark. You can dose 1 KH a week for the next three weeks.

Also, expect KH consumption to increase, as the stony corals start growing (if you have enough lighting and flow). When that happens you may need to measure KH more frequently.

 

[StrombusEyes]

Based on these measurements, your tank is consuming 0.3 KH per week. That is very little and in line with your current stocking.

So, nothing weird going on.
And a KH of 6.2 is not bad by itself. The ocean KH is about 6.5.

It is clear the water changes alone are not sufficient anymore.
At this point I would start dosing alkalinity weekly to add 0.3 KH using sodium bicarbonate.
To increase KH back to around the 8 mark. You can dose 1 KH a week for the next three weeks.

Also, expect KH consumption to increase, as the stony corals start growing (if you have enough lighting and flow). When that happens you may need to measure KH more frequently.

Thank you that’s really helpful!

 

[Randy Holmes-Farley Verified Community Expert]

I agree the demand is low and in line with the tank appearance.

An alk decline of 2 dKH total that goes into calcium carbonate would be associated with a 14 ppm drop in calcium, which you cannot readily detect with a kit (especially with small salinity changes impacting calcium more than that). While your calcium may be remaining in the normal range, it also is dropping at least as much as expected.

Magnesium depeltes very slowly and I'd stop measuring it. It will never drop faster than 1/10th as fast as calcium, and the large changes you observed are random test error (a big problem with magnesium testing).

Also, rising nitrate depletes alkalinity. A 50 ppm rise to nitrate depletes 2.23 dKH of alkalinity.