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A Small Scale Aquaponics System

In an earlier post Introduction to Aquaponics, I explained the basics of aquaponics. Today I’m going to describe the first small scale system I built a few years ago. To me, aquaponics is an interesting mixture of science and gardening. I have a mad scientist side so I decided to give aquaponics a try.

My typical approach to a new project is to start small. I look at the first attempt as a prototype. A way to get familiar with all aspects of the subject without going too big or too expensive. That was the approach I took to aquaponics.

I wanted to build a test system inside our hoop house which is only 9’ x 12’ so it had to be fairly small. The goal wasn’t to feed our family with this system – just to understand as many aspects of aquaponics as possible with a minimal investment.

The basic components of the system were grow beds, a fish tank, and a sump tank. There are numerous ways to configure a system, but they all have certain requirements in common.

Grow Beds
This is where your plants grow. Plants need access to water, oxygen, and nutrients. The most common type of grow bed is flood and drain. The plant roots get access to water and nutrients during the flooding, and oxygen during the draining. A flood and drain grow bed consists of grow media, a waterproof container to hold it, and something to control the flood and drain cycle.

For the grow beds, I used 12-gallon plastic bins. Ideally you want much larger grow beds so the timing of the flood and drain cycle is longer. These were fairly small, which was ok for the test system.

For the grow media, I used approximately 3/4” river rocks from the hardware store. They come in 40-50 pound bags. They need to be washed thoroughly before adding to the system to remove all the dirt and sand. Don’t use limestone-based products because they have a high pH and will affect the pH balance of your system.

Substantial Growth

Substantial Growth

The flood and drain cycle was controlled with a bell siphon. Check out this video for more information about how a bell siphon works. I built mine out of schedule 40 PVC. The stand pipe was attached to the bottom with a male threaded PVC fitting inserted through a hole I cut in the bottom of the bin. A female fitting was screwed down tight the hold it in place. I used some silicone sealant to make the seal watertight.

Minor leaks were not a big deal and here’s why. The drain lines underneath went straight down and released the water into a 4” PVC pipe I mounted horizontally with a slight slope toward the fish tank. Holes were cut in the top of the 4” pipe about the size of a baseball to allow room to add a tee. The tee slows the flow of water and actually helps start the siphon. Any small leaks went right into the 4” pipe so no water is lost.

The Fish Tank and the Sump Tank
For both the fish tank and the sump tank, I used 25 gallon heavy duty plastic bins. The idea of the sump tank is that it will handle the fluctuating water level caused by the flood and drain cycles so the level in fish tank doesn’t change. You want the level in the fish tank to be constant so you don’t stress the fish. You also want the water to be pulled from the bottom of the fish tank so the waste is circulated. Again, these requirements can be accomplished in many ways, but here is how I did it.

I cut a hole in the bottom of the fish tank and attached a standard PVC shower drain. Then the PVC pipe was routed up the outside of the tank and the discharge was into the sump tank. The height of the discharge controls the level in the fish tank. I failed to take pictures of this part so here is a sketch:

Fish Tank and Sump Tank Operation

Fish Tank and Sump Tank Operation

A pond pump was placed in the bottom of the sump tank and it was piped with 1” PVC back up to a manifold above the grow beds. Shut-off valves were added to each grow bed to control the flow rate of water.

Adjusting the Flood and Drain Cycles
The valves are used to fine tune the timing of the flood and drain cycle. Too little flow and the siphon will never start. The bed stays flooded and the plants get root rot. Too much flow and the siphon never breaks. The bed stays drained and the plants die from lack of water. In between is the sweet spot where the bell siphon functions properly and the bed alternates between the flooded and drained states.

Grow Beds and the Manifold

Grow Beds and the Manifold

While I was adjusting the system, I found that it was safer to keep the flow rate lower. Coming home from work and finding the bed stuck in the flooded state is better than it being drained. Plants can handle their roots staying wet longer than staying dry, especially in the hot climate.

A couple notes about the flood and drain bed system. First, the longer the cycle time the better. So much larger beds will work better than these small beds I built. The same flow rate will take much long to fill and drain. Second, large commercial systems can be based on electronically controlled valves which would allow you to program the timing. The bell siphon is a very low-cost way to automate the cycles with the only electrical input being the pump. But the bell siphon can be finicky so you’ll have to experiment with it.

Adding the Fish
I filled the fish tank with rainwater from our tanks, and added about 25 feeder goldfish. I wasn’t trying to grow fish to eat, and goldfish are very tolerant of wide temperature ranges. I started feeding them slowly at first then later as much as they would eat.

When the fish start to generate waste, ammonia builds up in the water. The high levels of ammonia promote the growth of bacteria that convert the ammonia to nitrites. Then the increased levels of nitrites attract another kind of bacteria that convert the nitrites into nitrates. The bacteria live on the surface of the grow media, so fairly small rocks provide a lot of surface area for the bacteria to colonize. When you first start up a new system, you’ll see a spike in ammonia then a drop off, following by a spike and drop off of nitrites, and finally increasing levels of nitrates.

Nitrates are what the plants feed on. Fish can handle much higher levels of nitrates than ammonia or nitrites. This is why you want to start feeding the fish slowly. You don’t want the ammonia levels too high because it could harm the fish. If you use cheap goldfish like I did, it’s probably an acceptable risk to ramp the feeding up faster.

Adding Plants
It often takes up to 30 days to get a system going enough to support plants. Since my system was experimental, I ramped up the feeding fast. I got test kits for ammonia, nitrites, and nitrates to monitor the levels. As soon as I saw the nitrate levels rising, I added the first plants. I started small with one each of various test plants: tomato, broccoli, squash, kale, lettuce, strawberry, pea, bean, and sweet potato.

It doesn’t make sense to grow a sweet potato in rocks but I wanted to see how the vines would do. I thought it might be a way to propagate vine cuttings for transplanting later into garden beds. The picture below shows the impressive root growth of the sweet potato.

Sweet Potato Roots - Impressive Growth

Sweet Potato Roots – Impressive Growth

What Did I Learn?
I have to say I was a bit shocked at how well some of the plants grew in this environment. Leafy green plants like kale and lettuces seemed to do very well. All of the plants had good leaf growth, but fruiting plants like tomatoes didn’t fruit as well as in soil.

Later in the Season

Later in the Season

There were some things I didn’t learn until later when I built a much larger system and I’ll cover that in more detail in later post. For example fruiting plants need phosphorous which they don’t get from fish waste. But too much phosphorous can cause greens like lettuce to go to seed quicker. While aquaponics seems to be better suited for growing greens, fruiting vegetables can work but are probably best grown in a separate system.

All plants need some other nutrients like iron that they don’t get from the fish waste, so you’ll have to learn about some of these additives. A great source of this information is a site called Bright Agrotech. They have a lot of really good videos that I found helpful.

I started the system in August and ran it until the plants died in December. When the first really cold weather hit, the plants that were close to the wall of my workshop survived because the wall had some retained heat. This told me that heating the greenhouse would allow a much longer growing season, possibly all through the winter.

When I built this first system, I had not embraced permaculture the way I have now. I didn’t know it at the time, but aquaponics follows some permaculture principles. The symbiotic relationship of the fish waste feeding the plants and the plants filtering the water for the fish is a form of function stacking. The extremely efficient use of water follows the principle of minimizing waste. Although aquaponics is not a natural system, I feel that it fits nicely in the permaculture mindset.

All in all, I thought it was a really cool experience. So much so that I eventually built a much larger system a coule years later. I’ll describe that one in the next post about aquaponics. If you have a little mad scientist in you like I do, consider trying a small aquaponics system of your own.

Resources for this post:
Descriptive Bell Siphon video: https://www.youtube.com/watch?v=CXaruS5z_Xc
Good site to understand plant nutrient needs: https://www.brightagrotech.com/

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