Some questions about salinity swings

[jgvergo]

I try to keep my specific gravity at 1.026, and I generally do a pretty good job of keeping it pegged at my target. However, some times stuff happens. For example, my skimmer runs wetter than I expected and my ATO adds freshwater in, resulting in a drop. Or, my refractometer might not be calibrated perfectly and the SG is too high.

When these things happen, I correct things "gently", i.e. over a period of hours or days. For example, if my SG drops below 1.026, I will change my ATO to use saltwater instead of freshwater (it is easy to do with my set up). If the SG is too high, I'll periodically dump some tank water and let the ATO replace it with fresh water.

Question 1: What is the acceptable SG range for normal operations?
Question 2: At what point is it no longer acceptable to slowly correct the salinity?
Question 3: How do the correction techniques changes as the deviation in SG gets greater?

 

[Anthony Wood]

Get your skimmer figured out and you won't have any more issues lol by doing all your doing is probably changing the level in the sump or tank and messing with how wet your skimmer is running. Also changing salinity is another reason your skimmer won't stay consistent. Your water viscosity is changing. Which is worsening your problems. Your salinity shouldn't change on an ATO.

Unless your using that smart level ATO which will let the water level drop quite a bit before it'll kick on. That would be a very possible explanation. When you check salinity is the level where the ATO sensor/float at the exact same level everytime?

Find the source that is causing the fluctuations that'll fix everything. It sounds like you have a smaller tank so a shift between 1.025-1.026 is acceptable just not ideal. But unless you have that smart level ATO that much of a fluctuation shouldn't happen. Turn off the skimmer for a day get your salinity at 1.026, make sure the water level where the ATO is is where you want it. Then turn on the skimmer and adjust it to the height in the body you want. Add SW back in until the level where your ATO is back up to where you want it. If your salinity changes from there you either need to get rid of the smart level ATO if that's what you have or the ATO float is in to turbulent water. Which bring another question what ATO do you have and where is the sensor/float placed?

 

[Anthony Wood]

Another note make sure the skimmer is in a spot that the water level doesn't change when the water level where your return in drops.

 

[jgvergo]

Get your skimmer figured out and you won't have any more issues lol by doing all your doing is probably changing the level in the sump or tank and messing with how wet your skimmer is running. Also changing salinity is another reason your skimmer won't stay consistent. Your water viscosity is changing. Which is worsening your problems. Your salinity shouldn't change on an ATO.

Unless your using that smart level ATO which will let the water level drop quite a bit before it'll kick on. That would be a very possible explanation. When you check salinity is the level where the ATO sensor/float at the exact same level everytime?

Find the source that is causing the fluctuations that'll fix everything. It sounds like you have a smaller tank so a shift between 1.025-1.026 is acceptable just not ideal. But unless you have that smart level ATO that much of a fluctuation shouldn't happen. Turn off the skimmer for a day get your salinity at 1.026, make sure the water level where the ATO is is where you want it. Then turn on the skimmer and adjust it to the height in the body you want. Add SW back in until the level where your ATO is back up to where you want it. If your salinity changes from there you either need to get rid of the smart level ATO if that's what you have or the ATO float is in to turbulent water. Which bring another question what ATO do you have and where is the sensor/float placed?

Anthony, I understand and I'm working to eliminate fluctuations. BUT, as I said, "stuff happens" I'm trying to understand when a fluctuation is within the normal range, a range that causes minor concerns and when it becomes a serious matter that demands immediate corrective action.

To your questions. My tank is 85 gallons and I have about 115 gallons in the system. My skimmer and ATO are DIY. Here are a few pictures:

The "ATO" is really a combination of a set of DIY float switches, my RO/DI and saltwater reservoirs and my Apex.

The water level stays between the two middle float switches, so I think there is less than a 1 gallon swing under normal top off operations.

 

[Randy Holmes-Farley]

IMO, the acceptable salinity range is very broad, luckily for us. Specific gravity from 1.023 to 1.028 certainly leads to wonderful reef tanks, but rapid changes are not desirable.

Here's a cut and paste of my comments on salinity:

Salinity

There are a variety of different ways to measure and report salinity, including conductivity probes, refractometers, and hydrometers. They typically report values for specific gravity (which has no units) or salinity (in units of ppt or parts per thousand, roughly corresponding to the number of grams of dry salt in 1 kg of the water), although conductivity (in units of mS/cm, milliSiemens per centimeter) is sometimes used.

Somewhat surprisingly, aquarists do not always use units that naturally follow from their measurement technique (specific gravity for hydrometers, refractive index for refractometers, and conductivity for conductivity probes) but rather use the units interchangeably.

For reference, natural ocean water has an average salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm. It often ranges from 34-36 ppt over reefs, but can be higher or lower locally for various reasons such as land run off of fresh water, or evaporation from a lagoon.

As far as I know, there is little real evidence that keeping a coral reef aquarium at anything other than natural levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. I have no idea if that is true or not, but I've not seen evidence that it is true. Substantial misunderstandings have also arisen in the past among aquarists as to how specific gravity really relates to salinity and density, especially considering temperature effects. For example, the density of seawater is less than the specific gravity, and measurements with glass hydrometers may require temperature correction, but newer devices do not need the aquarist to make corrections. Consequently, older salinity or specific gravity recommendations may not actually be referring to the same measurements that aquarists make today, even if the recommended numbers have been handed down

My recommendation is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a target salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).

 

[jgvergo]

IMO, the acceptable salinity range is very broad, luckily for us. Specific gravity from 1.023 to 1.028 certainly leads to wonderful reef tanks, but rapid changes are not desirable.

Here's a cut and paste of my comments on salinity:

Salinity

There are a variety of different ways to measure and report salinity, including conductivity probes, refractometers, and hydrometers. They typically report values for specific gravity (which has no units) or salinity (in units of ppt or parts per thousand, roughly corresponding to the number of grams of dry salt in 1 kg of the water), although conductivity (in units of mS/cm, milliSiemens per centimeter) is sometimes used.

Somewhat surprisingly, aquarists do not always use units that naturally follow from their measurement technique (specific gravity for hydrometers, refractive index for refractometers, and conductivity for conductivity probes) but rather use the units interchangeably.

For reference, natural ocean water has an average salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm. It often ranges from 34-36 ppt over reefs, but can be higher or lower locally for various reasons such as land run off of fresh water, or evaporation from a lagoon.

As far as I know, there is little real evidence that keeping a coral reef aquarium at anything other than natural levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. I have no idea if that is true or not, but I've not seen evidence that it is true. Substantial misunderstandings have also arisen in the past among aquarists as to how specific gravity really relates to salinity and density, especially considering temperature effects. For example, the density of seawater is less than the specific gravity, and measurements with glass hydrometers may require temperature correction, but newer devices do not need the aquarist to make corrections. Consequently, older salinity or specific gravity recommendations may not actually be referring to the same measurements that aquarists make today, even if the recommended numbers have been handed down

My recommendation is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a target salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).

Thanks Randy. I have read many of your articles, including the one you quoted here. That's why I target 1.026 as my ideal salinity. I don't believe I've ever been out of the 1.023 to 1.028 range, so I feel much more comfortable now, especially with my strategy of SLOWLY correcting the salinity back to 1.026. Much appreciated!

 

[fab]

...the density of seawater is less than the specific gravity...

Randy,
is this statement missing something or is it miswritten? What does it mean? Density and specific gravity are incommensurate, although related.
fab