Aeration Requirements In The Fish Tank And Troughs

If you don’t feel like reading this whole post, and just “want the facts”, here they are: If the water exiting your fish tank has a DO of 5 or above, you’re good. If the water exiting your troughs has a DO of 4 or above,

you’re good. If the water flowing into each trough has a minimum flow rate of 5 gallons a minute, you’re good. If you have higher DO’s and flow rate than this, you’re PAYING TOO MUCH FOR ELECTRICITY! Not important if you’re running a 20 square foot backyard system, but painful in the $2,000/month-too-much category if you’re running a 20,000 square foot commercial system.

(Below) A trough airstone properly adjusted: neither too much nor too little air.

IMPORTANT! If you want the best energy efficiency for your system you need to READ AND UNDERSTAND this post and the next, “Flow Rate In the Hydroponic Troughs” as completely as possible, because the results they create are interdependent and connected. If you are putting more aeration than necessary in your fish tank or troughs, or pumping more water than necessary, you will pay the extra cost forever. The materials lists for the six standard sizes of systems included with this manual have air pumps/blowers and water pumps that are already sized correctly for maximum energy efficiency, so don’t change them! If you design a custom system, you need to understand these two sections completely so you hit your target.

General requirements: BOTH the vegetables and the fish in aquaponics systems need more aeration in higher water temperatures, and less in lower water temperatures. Colder water absorbs oxygen more easily and holds it in solution longer; warmer water absorbs oxygen less easily and holds it in solution for a shorter time. Fish, being cold-blooded animals dependent on their environment for heat regulation, metabolize faster in warmer water and use more oxygen; and metabolize more slowly in colder water and use less oxygen. So statements such as “Fish need so many cubic feet per minute of air”, or even “you need supplemental aeration in your troughs” are incomplete and misleading because they don’t contain any reference to the factors which define oxygen usage and distribution in aquaponics systems: total weight and type of fish, amount and location(s) of aeration, water temperature, water flow rate in individual troughs, and length of the vegetable trough(s).

IMPORTANT! If you are using a cold-water fish and running colder water temperatures than we have (70-76°F) you will need to provide less aeration in both fish tanks and troughs. If you are using a warm-water fish and running warmer water temperatures than we have, you will need to provide more aeration in both fish tanks and troughs. If you’re in a location that’s hot in summer and cold in winter, you’ll have lower aeration requirments in the winter and higher aeration requirements in the summer; don’t assume because your system has plenty of aeration in winter that it will in summer!

IMPORTANT! There are five interdependent factors that influence when and how you aerate in the fish tanks and troughs, and what constitutes adequate aeration. The first one is amount and location of aeration supplied in fish tank, the second is amount and location of aeration in troughs, the third is water flow rate in troughs, the fourth is length of the troughs, and the fifth is water temperature. This discussion will illustrate the interplay between these factors:

The fish in the fish tank need oxygen, and so do the vegetables growing in the troughs. Plants need to be able to take up oxygen through their roots in order to live and grow, just as fish need oxygen. Don’t listen to people who tell you that vegetables DON’T need oxygen, but need carbon dioxide. This is true, for the LEAVES of the plants! However, the ROOTS of all plants need oxygen! This is why, when planting in the soil, sand or other soil amendments are often mixed in to provide air passages in the soil so the plant roots can get adequate oxygen.

How do you know how much aeration you have? Use your DO (Dissolved Oxygen) meter and dunk it in the water in question to find out (the DO strips are almost useless, sorry, and you can’t taste the water to measure DO!). For tilapia, 8 ppm DO is excellent, 7 is very good, 6 is good, 5 is decent, 4 is acceptable but not great, 3 is you have a problem and you need to figure it out quickly, 2 is you have a problem that you need to fix RIGHT NOW, and 1 means your fish are dying but you haven’t noticed it yet. If you start your system up in the summer, you may find that you are over-aerating during the winter when the water is colder, or vice-versa. If you start up in cold weather, make sure you still have enough aeration to produce good DO’s when the water gets warmer.

We built our first system the same as UVI’s: with airstones in the troughs, and the water flow from the single pump in the system divided up into two or three pairs of troughs. We built our second UVI-type system in a rush and didn’t complete installing the airstones in the troughs before it got planted. Then we noticed that things grew just as well in it with NO airstones in the troughs, as in our first system with airstones in the troughs. We checked our DO’s going into the troughs of this second system (5-6ppm) and then departing the troughs (they were only 0.5 to 0.75 ppm less), and concluded there was plenty of oxygen in the troughs for the plants without any additional airstones in the troughs. So we didn’t install airstones in any of the subsequent systems we built, and everything grew FINE, in three systems with water temperatures of 70-76°F, much colder than UVI systems.

But there was something sneaky happening here that we didn’t understand yet: In systems such as these, with no airstones in the troughs, the only oxygen going to the troughs is what comes in with the aerated water from the fish tank. As the water flows through the troughs it gradually loses oxygen as the plants and biological processes use it up, and if the trough is a long one, the DO can get so low that there’s not enough oxygen for the plants towards the end of the trough. If there’s enough aeration in the water coming out of the fish tank, if the trough is short enough, and if the water flow rate is high enough, then there’s good DO all the way to the end of the trough. We were unaware this was the situation we had when we started our investigation into trough aeration.

IMPORTANT! Why were we able to get away with no airstones in the troughs? We could omit these trough airstones with NO penalty on vegetable growth because we had a TON of aeration in the fish tanks, had 76-°F water, and were pumping it like MAD. These first systems were highly aerated (average DO in fish tanks 7-8) and pumped at a frenetic rate (20 gpm or MORE flow rate in each trough), and still had DO’s of 3-4 ppm where the water exited the troughs, indicating that the entire trough had gotten at least that level of DO. The real proof was that the vegetables grew just as well in our second system with no trough aeration as in our first that had it.

However, our electric bill was still through the roof! When we looked for ways to improve energy efficiency on the farm, we reduced the water flow rate into one trough in a test system, keeping the water flow rate the SAME in the other trough in this system as a control. Although the DO of the water coming into both these troughs was the same (in the 7’s) we noticed that the further you got down the test trough from the only aeration in the system (the airstones in the fish tank), the vegetables weren’t growing as well and the DO was quite a bit lower, in the 2’s and 3’s in fact.

Why? Even though the DO coming into this trough was good, the water was now flowing so slowly that the plants used up the oxygen quickly and depleted it before it got to the plants at the end of the trough. When we measured and found increasingly low DO’s along the trough, we added airstones to the test trough with the reduced water flow, and got excellent DO readings in the 4-5 ppm range at the exit end of the test trough. Over the next two weeks, the vegetable growth in the test trough was phenomenal compared to the “control” trough in the SAME system, which had the same system water, the original high water flow rate, good vegetable growth, the same DO of water entering the trough in the 7’s, and DO’s exiting the trough in the 3 ppm range.

We made some estimates of how much energy it took to power the new airstones in the test trough, and how much energy we would save by pumping water at the reduced flow rate, and concluded that we could save money by reducing water flow rate in the system troughs and adding a small amount of additional aeration in the troughs. After coming to this conclusion, we installed airstones in all the troughs in all our systems, and changed our pumps around to reduce water flow rate.

We refined this experiment over the next few months, and came up with some numbers: the tilapia did fine with DO’s of 4 or above, so we set our goal for the water exiting the fish tank at 5 ppm DO or more (but not too much more, right, because you’re paying for this in the electric bill). The plants did fine with DO’s of 3 or above, so we set our goal for the water exiting the troughs at 4 ppm DO or more. When we combined this with the information we got from our concurrent investigation of water flow rates in the troughs (next!), we found we could safely reduce both the amount of aeration supplied to the fish tank and the rate of pumping, thus saving us 40% energy overall compared to our previous scheme of aerating only in the fish tanks and pumping lots of water to get good DO’s in the troughs. This scheme put MORE oxygen in the water for LESS electrical consumption and money.

We confirmed this by running an experiment with a system that had good DO in the water exiting the fish tank, that had plenty of nutrients, that had water flow rate as low as 2 gpm through the troughs, but that had no airstones in the trough. We saw decreased growth in this test system. As soon as we added air to the troughs, plant growth exploded, indicating that the most influential limiting factor here was the lack of air in the troughs and not nutrient levels or flow rate.

HERE ARE THE NUMBERS: If the water exiting your fish tank has a DO of 5 or above, you’re good. If the water exiting your troughs has a DO of 4 or above, you’re good. If the water flowing into each trough has a minimum flow rate of 5 gallons a minute, you’re good. Read the other posts tagged with “DO, airstones, aeration, etc” to understand this subject thoroughly.