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Testing Salinity

We have our bucket of saltwater and we need to make sure the specific gravity is close to 1.025. Let's walk through that.

Calibrating the Milwaukee

First, we're going to calibrate the unit per the instructions. You did keep the manual right?

  • Place the Milwaukee on a level surface in a room with plenty of good light and press the "ON/OFF" button once. It should initialize and display the battery level.

  • Look closely at the lens and make sure it is clean. If not, clean it with a glass cleaning cloth or a soft tissue and no chemicals.

  • Use one of the pipettes that came with the unit to draw some distilled water from the provided vial.

  • Drop the distilled water into the sample well, which is surrounded by a shiny ring, until the water covers the lens. It takes four or five drops. Make sure there are no bubbles.

  • Press the "ZERO" button once. If all goes well, no errors will appear on the screen and the unit will be calibrated.

  • Gently absorb the distilled water from the lens with a soft tissue. Give the lens a wipe to make sure it is dry.

Always keep it clean and dry when you're done - it'll last longer. Also, rinse the pipette and give it a good shake to get it as dry as possible.


The process is the same, but instead of using distilled water, we use our the water we mixed. I prefer to have a pipette dedicated to distilled water and another for saltwater.

Drop the sample water over the lens and press the "READ" button. This is what I got:

Oops! We completely overshot the salinity!

The 15 cups of salt turned out to be way too much. We can use this as a teachable moment. Doing a bit of math, the correct amount would have been closer to 9 cups, which is about 1/3 cup per gallon. Next time, we'll try that amount instead.

To be sure that this was not an error, I cleaned the Milwaukee and measured a sample of water from one of my other tanks and it had the expected 1.025 sg.


So, how do we get the salinity down to our target of 1.025 sg? The simple answer is to add freshwater but how much do we add?

Fortunately, there are several online calculators that can answer this question. One such calculator is Hamza's Reef Saltwater Dilution Calculator.

  • First it prompts for the saltwater volume, which is 30 gallons in our case.

  • Next, it asks for the current salinity, which we measured to be 1.032 sg. Make sure to set the drop down to the right units.

  • Finally, we enter the desired salinity, which is 1.025 sg and swicth the units.

After clicking on the "Calculate" button, we get a result of 8.33 gallons. This is how much freshwater we have to add to reach 1.025 sg. So, let's do that.

I'm going to be conservative and start with just 6 gallons. This takes 18 minutes with my RO/DI system. I'll keep the pump and the powerhead going, so that the water continues to mix and then, I'll measure again...

After adding 6 gallons, I measured again and it came out to be 1.027 sg which is good progress. So, we'll add another 2 gallons and see where we end up...

That's much better! It's very satisfying when things work out.

Cross Check

We have what we believe is perfect saltwater ready for our tank but, not so fast: our instruments could be lying to us. It is always a good idea to cross check our results and that's why I recommend having at least two different ways to measure.

We're going to try the Red Sea Refractometer now to double-check. If you want to, take a look at the manual.

  • Take it out of its case and lift up the plastic cover.

  • Use a soft tissue or a glass cleaning cloth to clean the lens thoroughly.

  • Use the provided pipette to draw a sample of the saltwater, we just need a few drops.

  • Let the sample sit for a whole minute in the pipette, so that the temperature approaches room temperature.

  • Now squeeze the pipette to place about 6 to 8 drops on the lens.

  • Close the plastic cover. It's ok if some the water drips out of the side, but it's important that the lens is covered completely: make sure there are no dry patches on it. If so, lift the cover and add more water.

  • Finally, look into the eye piece. You will see the scale will be divided into a blue area and a white area. Where the two areas meet is the line that shows us the specific gravity.

I got a value of 1.025, which confirms the reading from the Milwaukee. We can be fairly certain that it is correct!

When it's Wrong

What do we do if the cross check doesn't give us the same result?

I always start by assuming that I did something wrong. I run the tests again paying close attention to the instructions and cleanliness.

If we still get a mismatch, we can go back and calibrate the instruments again and test one more time.

If results don't line up after calibration, we can look at the magnitude of the discrepancy and the advertised accuracy of the instruments. For example, the Milwaukee claims an accuracy of +- 0.002 sg. So, the actual specific gravity of our sample could be as much as 1.027 or as little as 1.023. If the reading from the Red Sea Refractometer is within that range, we can assume we're in the ballpark and move forward.

If all else fails, since we're filling up the tank for the first time and there are no living things involved, we can give ourselves a break and just go ahead and use the water we mixed. We could set aside a sample and take it to a local fish store later so they can test the salinity and give us another data point.

Now, we're ready to get this tank wet!