Help with dropping alk, low cal and mag

[adamf83]

For the past 2 days my alk has been steadily dropping (from 8.62 to 8.35) despite dosing and I’ve just carried out a water test and have lower than expected cal and mag (cal @ 363, mag @ 1000).

The alk is measured by a Reef Factory KH Keeper Plus, 4x per day.
The cal was measured using a Hanna and verified using the Red Sea Pro test kit.
The mag was measured using a Red Sea Pro test kit, but I have nothing to verify it with.

I don’t actually have any corals (had some previously which died) as I’ve been working on getting my nitrates and phosphates in a more suitable range before I try again, but I’m struggling to understand why I’ve been getting a steady decline of my alk measurement as since getting the KH Keeper Plus I’ve been able to maintain stability much better.

The tank is 145l (125l DT, ~20l sump) with just fish and a collection of bubble tip anemones.

Just wondering what to do to bring these 3 back into normal ranges? Is it just a case of simply dosing them individually, something like all for reef or something else? Also, what could have caused a decline of these 3?

Any advice would be appreciated.

 

[gbroadbridge]

Initially, I'd check the Alk using a different method.

I'd also recheck the pH probe calibration as a new probe can drift.

 

[adamf83]

Initially, I'd check the Alk using a different method.

I'd also recheck the pH probe calibration as a new probe can drift.

I’ve just checked it on the Red Sea Pro Alk test and it’s reading around 8.4 which is pretty close to the KH Keeper Plus.

I did only calibrate it a week ago, but I can do it again.

 

[Randy Holmes-Farley]

I'm suspicious that either the salinity is low, or the magnesium test is not accurate (quite common). It's hard to get magnesium that low.

What is the salinity and how are you measuring it?

 

[adamf83]

I’ve just checked it using a refractometer and it looks like 1.020. I usually use a Hanna checker but it needs calibrating.

Given what the salinity is reading at, what’s the safest way to raise that back to 1.026?

 

[Randy Holmes-Farley]

I’ve just checked it using a refractometer and it looks like 1.020. I usually use a Hanna checker but it needs calibrating.

Given what the salinity is reading at, what’s the safest way to raise that back to 1.026?

Replace evaporated water with new salt water until you get there.

 

[adamf83]

Would the low salinity also explain my lower than expected calcium reading?

 

[Randy Holmes-Farley]

Would the low salinity also explain my lower than expected calcium reading?

Yes.

Raising sg from 1.020 to 1.0264 gives a 32% rise in everything:

360 ppm calcium -->475 ppm

1000 ppm Mg --> 1320 ppm