Salinity Drop Mystery

[GRegg1720]

I have a 180 gal reef tank that has been up for a year. Recently I started having salt level drops. The tank will be find for several weeks then have sudden drops in salinity.
I have no leaks.
Salinity drop is not linked to high protein skimmer output.
I use RO water, and have an automated top off system that does not appear to be adding excessive amounts of RO water in response to evaporation loss.
I am using a computerized controller and have cleaned the salinity probe and confirmed its reading with a hydrometer. I get the same reading from both.
Since this morning I have added about 10 cups of Reef Crystal salt mix with no change in salinity.

So what could be causing this? I am showing drops on the order of 1.021 to 1.016 over night when this happens.

 

[Jimbo7]

The only thing I can think of is the ATO. I would unplug it for a couple nights and see if it is still happening. You might also want to mix up 5 gallon buckets of higher level salinity then add that. It's possible the salt mix is getting caught somewhere and not dissolving as fast as normal, plus I don't know if you have a sump or not, but adding salt to the DT can burn your coral and make your fish sick.

 

[David S]

same as above,do you no how much top up is being used as this will drop your salinity but I would have thought this would only happen over a long period without PWC

 

[austinsdad]

You need a refractometer. Much more precise and dependable. I'm not convinced you're getting good readings with either the hydrometer or your probe.

And be careful trying to adjust your salinty when it maybe doesn't need adjusting after all.