Key Elements of Water Quality #3 Section 1 - AERATION

This is part 3 of our "Key Elements to Water Quality" blog postings. I suggest that you go back to read the others as well on “Purity” and “Temperature.”  We are now going to be discussing aeration.  I will be dividing this into two sections due to it being such a vast subject.

Dissolved Oxygen (DO) can be a common thing that many overlooks and don’t give it the attention it deserves.  Also, when they think about it, they will only consider the fish needing oxygen but do not realize nitrifying bacteria and plant roots need it as much. 

When building a system, you need to consider ways to introduce oxygen into the system; that is called aeration.  Oxygen dissolves directly into the water surface from the atmosphere through natural conditions, fish can survive in such water.  This amount of DO diffusion does not meet the demands of an aquaponics system.  It needs additional oxygen in the water to offset the deficit created by the breakdown of waste (fish fecal matter or food), higher temperatures, and consumption by fish, nitrifying bacteria, plants, or any other organisms living in the water such as algae.
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Aeration can be performed many ways and we discuss that in the next article.  This blog post, I want you to become more familiar with dissolved oxygen and the effects it has on the system.  When we talk about DO, we’re talking about the oxygen that is soluble and has dissolved in the water.  It is measured by ppm (parts per million) or some will refer it to mg/L (milligrams per liter), which can be used interchangeably.

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​Low DO levels are not usually a problem with hobby aquaponic growers with low fish stocking rates. The problem tends to arise more in commercial operations with high stocking rates.  The systems featured in our “Step by Step Aquaponics” material are designed in such a way to ensure high levels of DO.  If you feel you are experiencing lower levels, there is no risk of adding too much oxygen through more aeration; when the water becomes saturated, the extra oxygen will disperse into the atmosphere. 

Testing for DO levels can require you to invest in expensive electronic equipment and this may be impractical for small aquaponics hobbyist.  By a quick scan on Amazon, you can find meters ranging from $150 to $800.  There are some test kits less than $100, these are not electronic items but use test tubes, droppers, and measuring charts.  One could consider these as well.   

​There are a lot of different variables that affect DO.  The salinity of the water can lower DO, but that’s not usually a problem for most aquaponic growers because you’re dealing with fresh water with low salinity.  More concerning is water temperature or the stocking density of the fish and sometimes, you may need to consider if you are overfeeding your fish or you have algae growth (you should NOT have algae in your system). 

​Our recommendation of stocking density is around 1 fish per 5 gallons of water or 20 kg of fish for every 1000 liters of water.  When making these calculations, I am only considering the fish tank size.  I realize that this may be a conservative approach, but I would rather be cautious.  As the system matures, it is safe to increase your stocking density, but you need to continue to monitor the health and wellness of your system by checking your pH, ammonia, nitrites, nitrates, plant growth, and that your fish are not gasping for air on the surface of the water.  If you see your fish doing this, you have a DO problem and it needs to be rectified immediately.  Click "READ MORE" in lower right to read the rest of the article.

​We have covered ways to adjust your water temperature in a previous blog post.  You need to ensure you monitor this.  Not only does it affect your fish, plants, and nitrifying bacteria, it dramatically lowers the DO.  As the water temperature rises, the solubility of oxygen decreases. Put it another way, the capacity of water to hold DO decreases as temperature increases; warmer water holds less oxygen than does cold water.  It is recommended that you increase aeration in warmer locations or as temperatures increase during certain times of the year, especially for certain breeds of fish.  Also, as temperatures rise, your organism growth increases dramatically, in return the oxygen consumption will rise with organisms alone. 

​Warm-water fish (e.g., bass, bluegill, and catfish) require about 5 ppm and cold-water fish (e.g., trout) require about 6.5 ppm of DO to ensure proper health and growth.  Even though Tilapia are tolerant of lower levels of DO, their growth rate will still be affected.  Fish will come to the surface of the water for oxygen if DO levels go down to 1 ppm.  It is recommended that DO levels be maintained at 5 ppm or higher in aquaponics systems.  If possible, oxygen levels should be measured frequently in a new system, but once procedures become standardized (e.g., proper fish stocking and feeding rates are determined, enough aeration is provided) it will not be necessary to measure DO as often.

Make sure you sign up for our blog.  We will be posting the next section on aeration soon.  You won't want to miss it.  We will be discussing ways to ensure you are incorporating proper aeration in your aquaponics system.  What equipment can you purchase and simple steps of design.

Shawn Paul
MorningStar Aquaponics