Salinity.. LOWERING?

[ClownieTang]

I have 100g with Live Rock and Live Sand. It has 4 chromis in the tank and 2 shrimp. I have been testing my salinity levels daily now and I have found a DECREASE in salinity levels! Why?? It is a rimless tank, water evaporates, I use to add filtered water to the tank because only water evaporates.. not salt. So last week, I saw the salinity level at 1.021. I would say about 3-4 gallons of water has evaporated.. or mysteriously gone somewhere in this week. and instead of adding filtered water, I added Reverse Osmosis Saltwater into the tank to increase the salinity level. So after I used up 4 gallons of RO saltwater, I tested the salinity, and it is 1.020+! I am using a CORALIFE Deep Six Hydrometer. I added my filtered water to it and the needle pointed to the bottom, so I know the Hydrometer is working. Why isn't the salinity increasing atleast? why would it drop for any reason??

[Reefing Madness]

I have 100g with Live Rock and Live Sand. It has 4 chromis in the tank and 2 shrimp. I have been testing my salinity levels daily now and I have found a DECREASE in salinity levels! Why?? It is a rimless tank, water evaporates, I use to add filtered water to the tank because only water evaporates.. not salt. So last week, I saw the salinity level at 1.021. I would say about 3-4 gallons of water has evaporated.. or mysteriously gone somewhere in this week. and instead of adding filtered water, I added Reverse Osmosis Saltwater into the tank to increase the salinity level. So after I used up 4 gallons of RO saltwater, I tested the salinity, and it is 1.020+! I am using a CORALIFE Deep Six Hydrometer. I added my filtered water to it and the needle pointed to the bottom, so I know the Hydrometer is working. Why isn't the salinity increasing atleast? why would it drop for any reason??

Hydrometers are not known for being very acurate. Take 5 readings, are they all the same? When taking a reading using one, you should get it wet a couple times, then stick it med way into the tank upside down, the once in the middle, tip up to fill, then read.
It could run low if you are wet skimming and taking water out of the tank that way.

[chrisfont23]

Hydrometers with the swing arm are incredibly unreliable. I would invest in a refractometer. I also lose about 3-4 g a week too. When you top off, you do it w/ freshwater - right? If so, that's basically everything there is and everything I can think of. IMO, the hydrometer is the problem.

[FoMoCo Master Tech]

Hydrometers get a bad rep... They make great door stops and bath toys for kids . Seriously, on your way to pick up a decent refractometer you can take a sample of your tank's water for the LFS to test for you. That way, you can compare the salinity with what you measured.

[TimH07]

Quoting myself from a post in my local club....


"The problem with hydrometer's are they are supposed to be calibrated at 60F but most will vary resulting in incorrect results. Specific gravity ratios are 1 part water per, say, .026 dissolved solids.

The problem is water expands with heat making it less dense even at subtle temperature variations. This is why we use parts per thousands of salt to measure more accurately.

I don't have exact numbers so this will be just to get an idea but...

A SG of 1.025 at 80 degrees cooling off to 75 degrees means the water per cubic cm becomes more dense, as the water contracts but the solids do not this means there is now more water per cm2 so the specific gravity changes to something like 1.024.

Specific gravity and salinity are not the same thing, which drives me nuts they print both on a hydrometer.

Salinity is an actual measurement of how much salt per thousand is in the water, in a 1 cubic cm block if the water is 35 ppt at 70 degrees its 35 ppt at 80 degrees but the SG changes by .002

NSW has a SG of 1.0265 on average and a salinity of 35 ppt"

[FoMoCo Master Tech]

I did not know about the relationship (or lack thereof) between sg and salinity. Thanks for the info. Question... Is there a similar accuracy variance when comparing ppt to psu? I see most scientific articles display primarily in ppt.