Testing The Waters

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The photo above shows a recent dissolved oxygen test of my catfish pond. There are all kinds of water test kits out there and they have gotten more user friendly in the past decade. This kit uses small glass ampules that are filled with an indicator solution. When mixed with a water sample, the indicator will change color to "indicate" the amount of dissolved oxygen present. There is also a partial vaccuum in the ampule.

To test a sample, you fill a small vial with 25mL of the water to be tested. Then the ampule is inserted into the vial with the long skinny end of the ampule down in the vial. A little pressure breaks the sealed tip of the ampule allowing the right amount of your water sample to be pushed into the ampule due to the slight vaccuum in the ampule.
The ampule is then inverted a few times to mix things. Wait two minutes and then compare the color in the ampule to the color comparison set.

There's some judgement at this point depending on how color blind you are...but essentially the comparison tube closest to your sample color will tell you dissolved oxygen in ppm...parts per million.

My sample last Saturday at midmorning seemed to be between 6 and 8 ppm. The fish seemed happy and active with this level.

In a pond, oxygen levels will rise and fall with time of day. Typically, dissolved oxygen is highest during the peak photosynthesis hours and drops off at night when oxygen is being used, but photosynthesis has gone into idle. If you are going to have a low oxygen fish kill in your pond, it will probably happen in the morning since the fish were using it all night even though little oxygen was being released by plants in the dark.
The graph below (which I borrowed from the University of Florida/IFAS program) shows how oxygen levels change diurnally.












This applies to goldfish bowls with a fish and a plant, backyard goldfish ponds in Washington state or Pinellas county, empty (except for otters) fish ponds in Alabama, a frozen headwater lake in Michigan, aquaculture wannabe ponds in Florida,reservoirs in northern California, frozen ponds in Illinois or New York, and even potential ponds in Missouri.

One thing to remember is, plants do not make oxygen. In photosynthesis, the plant is trying to make glucose (sugar). To make a molecule of glucose, you need 6 atoms of Carbon, 12 atoms of Hydrogen, and 6 atoms of Oxygen. Photosynthesis breaks water and carbon dioxide apart to reassemble the parts (C,H,O) into glucose. After the glucose is formed there are some extra Oxygen atoms lying around and these are released as waste.

It's as if I gave you a little pirate ship made of 36 leggo blocks (that would be the water and carbon dioxide) and said tear it apart and make something new from those 36 blocks. You decide to make a cool tower (glucose molecule), but you only needed 24 blocks to make the tower perfect...the extra 12 blocks (oxygen) weren't needed. They are essentially waste as are the oxygen atoms given off by the plant in photosynthesis.

The more light, the more photosynthesis, the more waste (oxygen) the plants produce. It's easy to see why Oxygen levels in the pond climb during the day and drop at night.

Photosynthesis ...it's like a miracle, but it's not. It's a little like magic, but it's not.

It's just some fellow organisms passing gas.

Breathe deeply.