One of the more recent aquaponic myths to circulate is the one that says that the contents of the fish tank must be turned over each hour.
It’s another one of those quaint notions that defies logic.
The fact is that the frequency with which the contents of a fish tank must be turned over are driven by circumstance rather than simplistic formula.
Some of the factors that determine the pumping rate include:
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the type of bio-filtration in use
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the pumping regime – flood and drain or continuous
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the aeration provisions
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whether sedimentary and suspended solids are removed.
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the amount of fish biomass.
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fish species
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water temperature
Keeping fish alive and healthy in a recirculating aquaculture (and aquaponics) system depends on the ability of the system to convert ammonia to nitrates, to rid itself of harmful gases (like carbon dioxide)….and to meet the oxygen needs of the fish.
That’s what has to happen and that’s what drives the volume of water that has to be moved through a system.
At the risk of repeating myself, ”simple guidelines” without qualification (at which point they cease to be simple) are unhelpful and are prospectively dangerous.
-o0o-
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Hi gary,
I have spent the last little while today reading your blogs about common “mythconceptions” and what you say seems to make a lot of sense. Is there any chance you could back up your claims with some hard evidence instead of just what your assumptions are. That may seem a bit harsh to say and i don’t want you to get the wrong idea. I’m just curious where you got your evidence to make your claims against the “other” people in aquaponics that disagree with you
noah…….as the mythconceptions posts suggest, there are lots of things about which other aquaponicists and I disagree so I’d ask that you be a bit more specific.
By way of a general response, most of my mythconception claims are a matter of science (as distinct from belief) and others are a matter of simple logic. All of them are supported by my experience or that of others.
If you can identify specific issues, I’d be happy to provide a more detailed response.
Hi Gary
Actually, it is turning over the entire system water per hour NOT the fish tank water volume. Thinking otherwise is a little dangerous. It is an accurate assessment if the “myth” is actually turning the TANK over per hour.
As we know the hydroponic plant volume of most systems is bigger than the aquaculture fish volume. For example, if you have 10,000 aquaculture and 20,000 liter raft system and use this “turn the fish tank over once per hour” which applies a flow rate of 10,000 per hour in fact will only turn the fish tank over once every three hours. Flow rate guideline would be a minimum of 30,000 liters per hour..
From a basic standpoint turning the water over in the entire system (including pipework) once per hour is quite a safe guideline. I don’t think it is very useful to anyone to suggest something is a “myth” without providing an example of the formula (which there is several) or an alternate solution for people to make their own calculations and come to their own conclusions. However, to suggest the calculations required to design the flow rate of a system is complicated is an accurate summary.
The main considerations for flow rate are TAN and Solids removal which are dictated by the feed used. Yes, there are other equations related to feed to calculate these two. But it is the TAN and Solids removal that will dictate the flow rate.
TAN is directly to the protein level in feed and fish excretion.
The solids generated is related to feed rate.
The feed rate is related to feed conversion ratio and specific growth rate.
These are related to metabolic rate.
and so on we go….
Considering the current aquaponic methods rely on the grow beds as their bio filter and solids filter the TAN and solids equations become rather pointless. In a backyard environment without the use of adequate filtration a general rule of thumb can only be used.
The use of filtration is another conversation all together related to your bio mass. Perhaps Gary would like to do a little myth busting on that issue?
Oxygen is another equation all together and something majority of people not involved in its use will not know and it is quite complicated. For example calculating the oxygen consumption is just like the above two consideration and it related to the feed used. But if you are looking for a starting point to play with is an average of 0.9kg of O2 is generated with diffusers per killowatt/hour. If you know what your fish demand is and your bio demand is you can start to calculate the amount of air you need to some degree. Confused yet?
The good news is there’s another one of those “mythical numbers” that are used in general terms which result in sufficient aeration with the use of diffusers. Even at densities up to 40kg/m3. I hesitate to provide this as it may be viewed as a snake oil salesman’s magical solution.
So, suggesting a turn over rate of the ENTIRE SYSTEM WATER ONCE PER HOUR is an adequate MINIMUM to work from. It is a good idea to oversize your pump in these backyard systems so the flow rate can be increased as needed (can always choke pumps back).
Two smaller pumps are a better idea so if one breaks down, you have at least some flow rate until you get a new one. The chances of them both breaking down at the same time are rare. The alternative is to have two of the same size pump, plumbed in line in case one breaks. Then you can alternate pump loads between the two.
Cheers
Paul V