Backyard Aquaponics
A place to discuss aquaponics

In the AP world you see many different styles of tanks used to hold fish. Other than the fish and that they hold water they tend to have one other thing in common. That is that they can and do work.

What follows is not holy writ that can not be ignored but rather a quick summary of how AQ tanks are designed and operated. While using this information will help you get better results for/from your fish with less work not using this information will not mean that your AP system will fail. It just won't be as good as it otherwise could be. AQ tanks are designed the way they are for very simple and very good reasons and the latest AQ tank design represent the best tank technology. Having said that there are sometimes good reasons for not using "best practice" designs or methods. For example IBCs make a cheap and effective tank for a small system but compared to a decent round tank they are pretty poor. Still for a small backyard system they may be the best choice. Also knowing how round tanks work can help you to make the most of an IBC as a tank.

Many of those tanks you see in the AP world are not as good as they could be*. Even if they are decent tanks the way they are installed or operated is also often not as good as they could be*. Which isn't to say that they don't or won't work its just that with a little knowledge from the experience of AQ they could be so much better. Even the vanted SLO design, while effectively removing the solids from many tanks in many systems, is pretty poor compared to the excellent results you can get with a decent AQ drain.

Round tanks have become the standard tank design throughout the recirculating aquaculture world. While many installations still use old designs it is widely excepted that the round tank is the best available tank within which to raise fish**. The reason for this is that it is very easy to maintain excellent water quality in a round tank and they are cheaper to run.

In Raceways and ponds water quality can be very variable. Excellent water next to the inlet, good water quality close to the inlet and the further you get from the inlet the worse it gets. Round tanks on the other hand tend to have very uniform water quality throughout the tank. Now as long as the tank is operated well then this mean excellent water quality at the inlet means pretty much you will have excellent water quality through out the tank. Of course bad water quality at the inlet means bad water quality every where but if you have a system where a round tank would have bad water quality then it would be even worse with a raceway.

The reason water quality is uniform in a round tank is because of the mixing action of the inlets, tank hydraulics and the drains.

Round tanks are installed so that a circular current is induced in the tank water. This current is called the rotating flow and it is created primarily by the water jetting into the tank from the inlet. Conveniently due to the friction between the water and the tank wall a second current is setup called a radial current****.


In standard***** AQ design it is not the rotating current you care about it is the radial current because that is the current that concentrates and transports the solids to the center of the tank. It is the radial current that travels along the floor of the tank towards the center in a spiral like path. The solids settle towards the bottom of the tank and are then concentrated towards the center of the tank as the spiral currents moving along the floor of the tank wind into the middle of the tank. They are then removed form the tank via a center drain.

In the original round tank design all the water was removed from a single center drain. The mark II design was the dual drain tank but since both drains were located in the center of the tank once the turn over rate reached a certain point the rotating and radial currents combined**** to create an area which was not fish friendly.


The mark III design is the Cornell Dual Drain tank. The picture above illustrates what happens when too much water goes out the center drain or drains. In the picture the tank has a side drain but the flow through the tank needs to be adjusted so the more water is flowing through the side drain. In a backyard setting you may not have enough flow to create this effect but if you do you can add a side drain which you can see in the side of the tank at the bottom of the picture. This doesn't have to be a box arrangement like in the photo it can just be another pipe through the wall of the tank.

The Cornell Dual Drain Tank has become the standard tank although you still see some people installing MII dual drain tanks. In a commercial AQ system only about 5%-10% of the total flow goes through the center drain. This is a major advantage in commercial AQ because that means that the solids are concentrated in a relatively small amount of flow. Which means smaller, cheaper filters which is a massive advantage of a dual drain tank. In a backyard system it most likely doesn't matter how much goes through the center versus the side (many people won't even need a side drain) as long as you are not creating a whirlpool. For people who feel the need for a SW or RFF by using a center drain they could use a much smaller vessel to create their SW or RFF. For those using GBs it doesn't really matter because all the flow should go through the beds anyway. If it doesn't it should go through a biofilter before going back to the FT.

Now if you don't have a round tank you can take the above on board and improve your system in a number of ways.

Say you have an IBC...

First thing to do is arrange the inlet so it jets the water along one side of the IBC or any other tank that is squarish for that matter. This is to setup the rotating flow. Commerical AQ inlets have a sparger on the inlet which is a pipe with a series of holes so that instead of one big jet there are a series of jets. This may be a simple vertical pipe so that there are jets the full depth of the tank or it may be a T-like arangement.


Second you need a drain in the center of your tank. You can put this through the bottom or you can have it coming in from the top (I'll take some photos of mine tomorrow if people want to see them.)

These two things will improve the water quality in your IBC. It still won't be as good as in a AQ tank because they are designed to have a width to depth ratio of more than 3:1. The depth to width ratio of an IBC is more like 1:1 and this means that the length of the spiral current is rather short so it doesn't really give the solids much time to settle to the bottom. This means that a greater proportion of them will be caught in the upward moving center current rather than being caught by the drain.












*Not as good as they could be = crap or relatively*** crap.

** There are some exceptions to this because some fish are unusaul (flounder for example) and this is a new tank design developed by Cornell that is probably going to be the new standard tank design but it is still being commercialised and its really another round tank anyway.

*** relative to how good they could be.

****In reality the water is not moving in two currents but one but it is easier to understand what is happening if you break it down into 2 dimensions rather than trying to picture it in you head in three.

*****there is good evidence that some fish(salmonids in paticular) prefer rotating currents and benefit from the speed of the current being increased. While has been known for some time to my knowledge it is not standard practice to vary the speed of the current to suit the size of fish and maximise the benefit to the fish. Rather the minimum rotating current to induce a radial current to keep the tank clean is all that is done.

Nice info Stuart.....

Was going to sticky it, but I think I need to un-sticky a few in the hardware section first.

I really enjoyed that read. Thanks Stuart.

Please do. I'd really like to see them (I'm hoping it somehow allows the drain to be in the centre of the IBC but still work as a SLO).

Really appreciate the effort you've gone to. I was very keen to get a round tank but the available area made it impossible, so anything I can do to improve the efficiency and effectiveness of an IBC tank would be great.

I will definitely implement the jet along one side suggestion in my new setup.

that's great stuff Stuart, what sort of water velocity should we be aiming at?

Should the velocity in the FT be designed to keep the Fish Waste in suspension?

my FT is about 4.5m diameter and 0.7 to 0.8 deep so its a bit under the 3:1 ratio

I have plumbed my return flows induce a rotation it goes around in approx 10minutes hardly a whirlpool?

I dropped a leaf in the water and it wondered around like a drunken sailor then got lost
but I guess it would be 0.1 rpm

that would make the rotating flow at about 0.23m/s?

interesting point on the whirlpool photo in your post compared to the radial/rotating flow diagrams,
I always imagined that in a whirlpool the surface at the centre is being sucked down thus the hollow in the middle of the whirlpool, in the radial flow diagram the water flows up at the centre?

Wow! I would have never thought the side outlet would handle the bulk of the water exiting the fish tank. It makes a lot of sense though. Also, I never knew what spargers were called, I always just called them a pipe with holes drilled on the side. (Usually in response to people asking me how to set up their bait tanks.) though the spargers you pictured looked like they had nozzles of some sort.

Good job Stuart!

Ron- the nozzles are for very large tanks and are adjustable so that they can be aimed at dead spots in the tank.

There is another type of tank design called a "mix cell raceway" that is used because of its efficient use of space.



You divide the raceway into equal lengths by installing center bottom drains and a flow manifold (or 1 water inlet and 3 airlifts) in each corner. This creates circular flowing "cells" within the raceway and eliminates plug flow that is common in rectangular raceways. Taking lunch so no time for sketchup but hopefully everyone gets the idea!

Not to mention that a lot of places already have raceways and this probably allows them to use their existing raceways.

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Great job Stuart

Yeah that is pretty weird isn't.

Actually the "hole" is not created by the water at all. It is created by air pressure.

Get your head round that one. It took me quite while.

The Mixed Cell Race Way is the M IV AQ tank. They were developed a few years by a guy called Watten for just the purpose of getting better water quality in old raceways. From Watten's original idea work has continued and as Ryan said they are brilliant because you get all the benefits of a round tanks water quality but also all the fish handling benefits of a race way, all the space saving benefits of a raceway and all the cheaper construction costs of a raceway.

All the commercial work I have done incorporates them as the FTs. Four years ago I had hoped that we would be one first commercial operations to use them but I know of commercial operations in the States and Spain that are rolling them out.


This shows a 3 cell raceway that was built for research to test among other things the velocity profiles within the water and the level of hydraulic mixing.


Not a great photo but it shows the MCRW from the diagram above.


The view shows the spargers and you can see that some have jets pointing toward the camera while others are facing across the tank.

According to most you want an average velocity of about 0.15m/s to make them self cleaning.

For all the research on salmonid aquaculture it is amazing how little information there is on swim optimum swim speeds. There are about 6 good articles that have published results of their studies and they tend to suggest that there is increased mortality in very young fish but reduced mortality in old fish. Better FCR, reduced oxygen demand*, better muslce development, better flesh (taste and texture). Lots o documented benefits but the studies are really limited. This many fish subjected to this regime for this many weeks. Absolutely no commercialisation of the science. In fact every AQ consultant I talked to said that it was dumb to design a system to incorporate it. This is one of the pieces of research that I want to do in my commercial systems.

The theoretical swim speeds are some where from .5 to 2 body lengths per second. The higher speeds were not constant but rather given in bursts. So for a 0.15m fish you want a speed of where between 0.075m/s-0.3m/s. The designs I've worked on will be able to vary the speeds within the tank and vary the speed as the fish grow and between cells so that different sized fish may be catered for within the one raceway. Basically small fish will go in at one end of the raceway and large ones will get harvested at the other.

* Yes I know that doesn't make intuitive sense but there you go. That is why scientists often run experiments even when they and everyone else knows what the answer will be because every now and then you find out that everyone's intuitive common sense thinking was COMPLETELY WRONG.

No problem Stuart, the pressure in any point in an open still body of water is the sum of the atmospheric pressure at the surface plus the pressure due to water depth. At the surface of the water there is 1 atm is about 1 bar, 100kPa or 10m of water head. So at a water depth of 10m the absolute pressure is 2 bar or 200kPa.

If you swirl water around centrifugal force tends to push the water outwards creating a lower pressure in the centre which lets the air pressure push the water down.

The other type of whirlpool is a large drain which allows the water to removed quicker than it can be replaced by the surrounding water body, thus the lower water pressure in the centre and the vortex.

Sometimes the atmospheric pressure is forgotten about when doing pressure calculations, its easy to overlook air pressure as it's always around us.

Suction or vacuum is weird because the less you have the more you have. but if you had a perfect vacuum the pressure acting on the vessel due to atmospheric pressure will be about 100kPa.

Well I know that now.

Where were you when I was trying to get my head around vortexes.

LOL, probably designing pump stations.

Quite good information!

I'm surprised they make commercial FT's out of wood and pond liner. I didn't think the life would be long enough to justify it.