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Specific Gravity Notes

Posted 01-23-2010 at 09:34 PM by NASAGeek
Updated 09-12-2010 at 05:02 PM by NASAGeek

Specific Gravity (SG)

Recommended Target Levels
-Suggested Reef Aquarium 1.023 - 1.025
-Suggested FOWLR Aquarium 1.020 - 1.025
-Natural Reef 1.025 Users
6.5% 1.022-1.023
64.5% 1.024-1.025
29% 1.026

Conclusion : Target 1.025

Parameter Measurement Options
1) Refractometers
2) Hydrometers
3) Conductivity Probes

Refractometers : Optical measurement. Place a drop of water on the refractometer surface, look through eyepiece and read value. Must be calibrated for temperature. Accurate, reliable, low cost.

Hydrometers : Either float hydrometers (built in with thermometer) or plastic hydrometers. Work based on density of water. Inaccurate, very cheap. Not recommended.

Conductivity Probe : Probes can be stand alone or attached to higher end Aquarium Controller. Accurate, limited life ( a few years), expensive (approx $90).

Conclusion : Refractometer.

Parameter Out of Range Isolation Steps
If SG is not at desired target, investigate the following isolation steps.

1) Automatic Top Off System : If ATO fails open putting excess RO/DI water in the tank, it would lower the SG. Water Level in Sump would be higher than normal. If ATO is out of water and evaporation has occured, it would raise the SG. Water level in the Sump would be lower than normal.

2) Water Changes not at Target Level. Ensure new water being added to tank is at the target level. Too high raises SG level, too low, lowers SG.

3) Water Change water not at desired target. Lack of circulation or evaporation could cause water change water to have changed salinity since prepared. Check SG before adding to tank.

4) Wet-Skimming, and other activities that cause your tank to lose water will gradually lower SG.

5) Check calibration of measurement device.

Once desired target is reached, SG should not varying greatly as long as Water Change Water is at target level.

Parameter Recovery Steps
If SG is below target,
Option #1) Add small amounts of salt to sump every few hours to gradually raise it. Roughly 1/2 cup per hour for a 75 gal total water volume system until 1.025 is reached.

Option #2) Add salt water to ATO. Will gradually raise SG as evaporated water is replaced.

Option #3) Perform Water Change with water at SG higher than target.

If SG is above target,
Option #1) Take some water out and replace with RO/DI every few hours to gradually reduce it. Roughly 1/2 gal salt water removed and 1/2 gal RO/DI replaced every hour for a 75 gal total water volume system until 1.025 is reached.

Option #2) Increase skimmate produced from skimming to induce greater use of ATO replentishment.

Option #3) Perform Water Change with water at SG lower than target.

1) Do not raise or lower SG by more than 0.001 per day.

Related Articles

Reef Aquarium Water Parameters by Randy Holmes-Farley -

Refractometers and Salinity Measurement

Temperature Corrections for Hydrometers

Reef Aquarium Salinity: Homemade Calibration Standards

Specific Gravity Measurement

Reef Aquarium Water Parameters (a summary general article)

What is TDS?
Posted in Water Chemistry
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  1. Old Comment
    NASAGeek's Avatar
    Additional comments provided by Screwtape

    Using 2 part calcium/alkalinity supplements will slowly raise SG over time from the byproducts (sodium and chloride ions) of the chemicals added. I don't think most people notice this if you do enough water changes but it's something to watch for, especially if you have a heavily stocked tank.

    Also a drop in salinity could be from a slow leak from the tank or plumbing. You would not notice any levels changing, just possibly a puddle, but you may not even see a puddle as it may be evaporating. Excess salt creep around plumbing joints/bulkheads etc may be a good indicator though.

    Also keep an eye on your calibration solution for the refractometer. Over time it can evaporate a little bit if you're not careful to reseal it immediately after use, or if you somehow contaminate it slowly over time with a dropper or whatever.
    Posted 01-24-2010 at 11:43 AM by NASAGeek NASAGeek is offline
  2. Old Comment
    NASAGeek's Avatar Thread on Specific Gravity
    Posted 01-24-2010 at 11:45 AM by NASAGeek NASAGeek is offline
  3. Old Comment
    Jason McKenzie's Avatar
    Great blog entry
    Posted 01-26-2010 at 06:28 PM by Jason McKenzie Jason McKenzie is offline
  4. Old Comment
    Posted 01-30-2010 at 11:36 AM by NASAGeek NASAGeek is offline
  5. Old Comment
    slocal's Avatar
    Very cool info. You'll have to keep in mind that the websites selling livestock will give basically the same info for any coral, fish and invert. This is to just cover all the bases. Many that run in the 1.026 range due so for algae control and because their specific livestock are comfortable with the salinity.
    Posted 03-03-2010 at 08:22 PM by slocal slocal is offline
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