Q & A: Testing Your Level of Dissolved Oxygen

Question: What is the highest oxygen level (ppm) that a saltwater tank can achieve? I tested my 30 gallon with a Salifert oxygen test kit and got a reading of 6 ppm. The highest on the test is 14 ppm. Will I be able to reach that maximum level in my tank using the Oxydator? Salifert recommends a reading of 7 ppm for a marine tank. I want to reach that maximum saturation level of 14 ppm in my tank.

Answer: The "highest oxygen level" that can be achieved in aquarium water is primarily a function of two (2) things: temperature and salinity.

The warmer the water and/or the higher the salinity, the lower the saturation capacity ("saturation" means "maxed out - cannot hold any more dissolved oxygen"), meaning relatively lower ppm oxygen numbers at saturation for warmer water and/or higher salinity.

Conversely, colder water and/or lower salinity can hold more dissolved oxygen, meaning relatively higher ppm oxygen numbers at saturation for colder water and/or lower salinity.

Thus, the "ppm oxygen" measurement displayed by an oxygen test is a meaningless number in and of itself - you must know both the temperature and the salinity of the sample that was tested, especially if you want to compare it with other tests of your water or that of a buddy's tank. For example, if your 6ppm was shown by real hot water at very high salinity, that could be 100% oxygen saturation! If your 6ppm was shown by very cold water at very low salinity, you'd be way less than 50% saturation. Got it?

So, to conduct a meaningful tests of your aquarium's dissolved oxygen levels over a given period of time, you must maintain both the temperature and the salinity of your aquarium's water at exactly the same levels for the duration of the test period to ensure that you can make a valid comparison of the numbers with one another -- so that they're not "skewed" by variances of either temperature or salinity or both (which would render the numbers essentially useless as to any kind of meaningful comparison analysis of ppm oxygen).

Of course, the fact that you'll be refilling the reservoir with solution more frequently early on, and less and less frequently as time goes by (as the water is getting closer and closer to oxygen saturation) is a pretty good indication that something good is happening there... Rest assured, you can't "overdose" your tank because he water can hold only so much oxygen (which you now realize is variable due to the influences of both temperature and salinity).

"Time-of-day" is also a variable that you want to be constant in addition to temperature and salinity for your testing to yield meaningful results.

Why? Because although plants and algae produce oxygen by day, they consume oxygen at night -- lower oxygen levels will occur during the night. So always measure your oxygen at the same time of day (but not immediately after lights-on, of course) to ensure results that you can compare with one another meaningfully.

I also wanted to make sure you knew what you were asking when you said, "I want to reach that maximum saturation level of 14 ppm in my tank" (apparently you desire this because that's the highest number Salifert's test can show). Just 'cause the air gauge at the gas station for the tires on your car goes up to 85psi doesn't mean your tires can hold that particular gauge's maximum measurable pressure!

What you really want is "maximum dissolved oxygen in the water" (as close to "saturation" as your water can get), irrespective of what any "test result" numbers may show (which, by now you realize are affected by temperature, salinity and time-of-day). Incidentally, the number and species of fish must also remain constant, so don't change that during the test period either.

Weekly oxygen test results are very useful for comparison purposes to see the "trend" showing the increase in the amount of oxygen in the water of a given tank by using an Oxydator but remember, the results are valid for comparison only when all other factors that can affect those results up or down (esp. temperature and salinity) are held constant.

Given two columns, Week Number and Test Result, Excel can produce a graph of that data at the touch of a button. If you were to log your results every week for a few months, the resulting graph would nicely display what you've achieved. I would be honored if somebody would make a blog entry showing that table and graph for all to see (including a note of the temperature, salinity, time-of-day and livestock constants).

Again, just to make sure you're not misunderstanding, a reading of 14 on Salifert's scale DOES NOT mean 100% oxygen saturation (that's why the scale shows values in "ppm" and not a "percentage"). You can be at 100% with a much lower reading than 14. In fact, 8 ppm can be 100%. It all depends (primarily) on temperature and salinity.

Here are some examples for you taken from Tetra's Oxygen Test Instruction Leaflet (they use a "color scale" method, in which results are shown in units of mg/l as the color gets darker and darker pink as oxygen mg/l increases):

Entitled "Saturation of Oxygen in seawater at different densities (values correspond to a 100% saturation):"

At 77 degrees Fahrenheit:
Salinity 1.018: 7.0 mg/l
Salinity 1.022: 6.8 mg/l
Salinity 1.026: 6.6 mg/l
Salinity 1.030: 6.4 mg/l

At 86 degrees Fahrenheit:
Salinity 1.018: 6.4 mg/l
Salinity 1.022: 6.2 mg/l
Salinity 1.026: 6.0 mg/l
Salinity 1.030: 5.8 mg/l

Each of the above "mg/l" represents 100% oxygen saturation at the stated temperature and salinity.

Meaning that 5.8 mg/l (at 86 degrees F and 1.030) AND 7.0 mg/l (at 77 degrees F and 1.018) BOTH represent 100% oxygen saturation - it's all relative to the temperature and salinity of the water.

Want to go off-scale high on Salifert's test? Test ice cold tap water with no salt in it (by definition, that water is at a very low temperature with very low salinity). You'll be over 14 because water in that condition has the ability to and is in fact holding LOTS of dissolved oxygen.

Then mix a lot of Instant Ocean or Reef Crystals into a small amount of boiling hot water (by definition, that water is at a very high temperature with very high salinity). I bet Salifert would barely register a 1 because water in that condition can hold, and is holding, only a very small amount of dissolved oxygen. We've all watched water boil. Ever wonder where all those bubbles are coming from? That's the dissolved oxygen being violently forced from the water because as temperature rises, it hits the point at which the "excess" dissolved oxygen (which the water could hold just fine at the lower temperature) is literally being "forced" out of the now-hotter water!

So now you know that 14 and 1 can BOTH represent 100% oxygen saturation - it all depends (primarily) on the temperature and salinity of the water being tested.