Salinity and kH keep dropping in a new tank

nikolav · May 23, 2026

Hi guys
I started my 80L / ~20 Gal nano reef a couple of months ago. Around two months ago I started adding some small fish and a month or so ago I starhed adding some LPS and lately some SPS corals. Totally 4 small fish and 6-7 corals in total. When I sgtarted adding the fish, I saw some diatomes developing, but too much.
Up untill a month ago everything looked good. Salinity was stable 1.025, kH 8, Ca and MG in the recomemnded digits. Zero nitrates and in the begining I didn't measure for sulphates.
Then lately I noticed salinity and alkalinity drop. Also the Ca and the Mg dropped a bit.
Today's numbers:
Salinity: 1.021 - 1.024 depending how I measure - see below
kH: 6.5 (up from 5.9 yesterday)
Ca: 370
Mg: 1260
Nitrates: 0
Phosphates: 0.07 (dropped from ~0.14 a week ago)
pH: 8.39

For the salinity I use a refractometer and a Hanna checker, both calibrated with 35 ppt solution. They both show 1.026 when I test freshly mixed see water (Aqua forest reef salt). When I test the aquarium water the refractometer, it shows 1.024, the Hanna checker shows 1.021. With both I wait a minute or two begore I get the reading. The ambient temperature in the house is 20-21C. The tank water is 25.5C

It's AIO tank and there are no leeks - confirmed. The skimmer only collected about 30-50 ml in a week. The ATO (just RO water, no kalkwasser) keeps it relatively steady. I see some up and down levels in the return chamber, but that's not more a cm and in volume not more than 70 - 100 ml. All ATO/dosing lines are above the water, so nothing can go back to a container. I'm excluding any of these as a factor.

I only started dosing kH using Randy's recipe a week ago. Witha dosing pump, I was dosing 1ml 12 times a day. The kH kept dropping. Yesterday I increased it to 2ml 12 times a day (24ml a day) and only now it shows some increase - from 5.9 to 6.5 in about 12 hours.
I just manually dosed some Ca and Mg (Andy's recipe) and will measure again tomorrow.

For now I'm reluctant of increasing the salinity as I'm not 100% sure these reedings are correct.

I have about 2 to 4 cm sand bed, about 10 kg of rock. The tank came with some filtration ceramic media and Aqua One's Phos-Nitra Reducer Beads.

So... Any ideas, guys?

slingfox · May 23, 2026

The key issue here is get a reliable way to measure your salinity. The Tropic Marine Hydrometer is the gold standard. Get a 500ml glass cylinder and fill it up with 400ml of saltwater to do the measurement. I prefer the VeeGee STX-3 since it is easier to use but it is also much more expensive.

In the short term you can check calibration on your current instruments as follows. I would not trust any of the off-the-shelf calibration solutions:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/

nikolav · May 23, 2026

This is what I used to calibrate my refractometer and to confirm the calibration of the hanna checker. The hanna checker was calibrated just before the check with its own 35 ppm calibration liquid.
I have no reason to doubt any of the meters.
The salt's instructions reads add 415 g per 10l for 1.026 sg, that is what I add any bothe instruments show 1.026 with the fresh salt water.
And I don't have access to any other checkers unfortunately.

T

Randy Holmes-Farley · May 23, 2026

Calibration with the Hanna is with what fluid? I do not have confidence in Hanna solution. I recommend making your own:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/
There’s no emergency here so I’m going to remove that tag from the thread.

nikolav · May 23, 2026

Randy, this is precisely what I did:
Dissolved 3.65 grams of sodium chloride (table salt?) in 96.35 grams (mL) of purified freshwater.
Sorry if I wasn't clear before.

With both calibration samples (the diy and the hanna one) and with the freshly mixed saltwater both instruments show 1.026. ONLY with the aquarium water they differ. The Hanna checker shows constantly ~ 0.002 lower SG.

I also forgot to mention. No salt creep, no visible precipitation on the heater or pump.

Privateye · May 23, 2026

Falling calcium, alkalinity, and magnesium is normal when keeping stony corals. The alkalinity will fall in any aquarium you feed, fresh or salt, over time.

The salinity is the confusing part though. If you're not adding any fresh water, then I'd expect the salinity to rise as your sump level drops from evaporation.

Randy Holmes-Farley · May 24, 2026

nikolav said:
Randy, this is precisely what I did:
Dissolved 3.65 grams of sodium chloride (table salt?) in 96.35 grams (mL) of purified freshwater.
Sorry if I wasn't clear before.

With both calibration samples (the diy and the hanna one) and with the freshly mixed saltwater both instruments show 1.026. ONLY with the aquarium water they differ. The Hanna checker shows constantly ~ 0.002 lower SG.

I also forgot to mention. No salt creep, no visible precipitation on the heater or pump.

3.65 g NaCl and 96.35 g water equates to 35 ppt seawater equivalent for a refractometer, but not for a conductivity meter like the Hanna.

For the Hanna, you need 3.29 weight percent sodium chloride to match 35 ppt seawater.

This article has all the details for both and why they are different:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/

nikolav · May 24, 2026

Thanks Randy. I missed that part, reading is hard, mate :)
I'll redo the Hanna calibration and report back.

nikolav · May 25, 2026

So, I made the conductivity sollution. Measured it with the Hanna checker and it showed 1.023 (I was going to swich the checker later to ppt, which is a pain, but didn't get there). Then I ran the calibration procedure and it failed - showed me some error message and that was it. Tried calibrateing with my last Hanna's 35 ppt solution, but that failed too. I'm pretty sure the new calibration was not recorded, but after that it measured the conductivity standard as 1.018 and the aquarium water as 1.017. I have some more of the Hanna's solutuion in the mail, but I guess I had enough of this Hanna product.

During the last couple of days I slowly increased the tank water to 1.026 using the calibrated refractometer and it's been a day w/out any further decline in salinity.

Randy Holmes-Farley · May 25, 2026

I don't know anything about why the Hanna might fail to calibrate with either my standard or the Hanna standard, short of a dying battery, but it may be time to stop using it.

nikolav · May 28, 2026

So, I think the salinity swings are fixed after a stopped using the hanna checher. Now I'm only using the calibrated refractometer and the salinity seems stable.
I manually bumped the Calcium and magnesium to about 420 and 1390 respectively and 4-5 days later they are about 405 and 1310. No idea of that's ok as a drop.
What still bothers me is the alkalinity. It's been swinging between 6.8 and 7.1 and I'm dosing currently 32 ml of Randy's recipe 1 solution. Isn't that too much for effectively 70l of water and just a few frags? I think coraline is starting to grow on the back wall, but I don't think I'm seeing any growth with the corals. Not that I'm expecting them to grow any faster. What am I missing here?

Randy Holmes-Farley · May 28, 2026

There is zero concern about alk varying from 6.8 to 7.1 dKH.

Your addition is about 2.3 dKH per day.

Do you notice and sand hardening or other signs of precipitation?

BryanM · May 28, 2026

nikolav said:
This is what I used to calibrate my refractometer and to confirm the calibration of the hanna checker. The hanna checker was calibrated just before the check with its own 35 ppm calibration liquid.
I have no reason to doubt any of the meters.
The salt's instructions reads add 415 g per 10l for 1.026 sg, that is what I add any bothe instruments show 1.026 with the fresh salt water.
And I don't have access to any other checkers unfortunately.

T

I had a Milwaukee digital salinity tester and calibration fluid, it always produced funky results.

I calibrated normal refractometers, both with calibration fluid or RODI, those always read spot on, send in ICP test, results come back 33 instead of 35.

I switched to the tropic marin hydrometer, adjusted my tanks salinity to 1.026/35, sent in an ICP, and it came back 1.026.

I don't trust anything else.

nikolav · May 28, 2026

Randy Holmes-Farley said:
There is zero concern about alk varying from 6.8 to 7.1 dKH.

Your addition is about 2.3 dKH per day.

Do you notice and sand hardening or other signs of precipitation?

I'm not 100% sure, but I don't think so. The heater and the pumps are spottles except for some algae. The sand is indeat a bit hard, but I thought this is due to the algae on it. Not rock hard, just some crust, which was very easy to break with aa turkey baster and now it's again clean and "fluffy". So not sure how to tell if it was the algae or precipitation.

Unless I'm mistaking precipitation for Coraline forming.