More often than not, I get asked what or even how should I build my aquaponics system in my garden and what plants can I grow, what fish can I use, how much should I feed them... These are the recurring questions in the aquaponics backyard garden world. With this article I will try to set the baseline of what and how an aquaponics system should come to be and hopefully bring you closer to success.
It is such a jungle out there that clearing the wrong from the right about aquaponics is overwhelming and a very difficult task.
Remember that rules are made to be broken and if you don't like what my gold ratios of aquaponics, you are free to experiment what works for you. I promise not to hold any grudge ;) On another note though, if you use rules that are not mentioned in this article, be aware that for me to help a system that has crashed due to not applying any of the rules stated below, I will not be able to help you unless you agree to start from scratch again and do things as I state them here.
I am no Aquaponics God and do not pretend to know it all but for the past two years I have been diligently studying this topic and have a acquired a certain expertise.
So without any further ado, let's jump right into it.
Different methods of doing aquaponics
Media beds are what I would recommend for any aquaponics beginners. It is very easy to install, operate and maintain. Why not NFT (Nutrient Film Techniques) or DWC (Deep Water Culture) you could ask?
One of the advantages of using media beds is because in itself it has the three stages of filtration:
And the obvious perks of using media bed is because you have the closes simulation of soil agriculture as your plants/veggies will grow in media.
So to summarise, media bed, not only has filtration but it also has:
Aquaponics (Media) Grow Beds
One of the first rule and very important question is: how deep does my grow bed has to be?
There is only one answer to that: a minimum of 30cm (12") and there are at least two reasons as of why that is...
First, it offers ideal depth for the primary and anchoring root and offers ideal conditions for the lateral roots in charge of all the yummy nutrition/filtration (and Nitrate filtration, more on this here).
And secondly, the surface area represented by the total media in your grow bed will determine how much ammonia you can filter and this, will define how many fish you can have.
This rule applies when using the feed rate: fish feed per grow bed area (m2) and it works for either leafy greens or fruiting plants. See feed rate section of this article.
The Fish Tank
The more water volume, the easier it is to keep the chemistry in check. Simply put, in small volumes changes occur quickly compared to large volumes. When adjusting your pH for example, with a large volume, I recommend to dose not only the appropriate acid or base solution but it has to be the right amount for the total volume. One mistake at this level will and can be corrected but in a small volume of water, a mistake can have dramatic consequences and can require a 100% water change. So if you can, go for large tank (1000lt volume) you won't regret it.
Aquarium systems are cute but very difficult to maintain without almost daily water changes.
The second aspect of choosing a fish tank would be what it is made of. I highly recommend using food grade materials and if going for plastic, make sure that it is stamped with the HDPE logo and better if it has a the logo of a fork, even better, it means it is made food grade.
Tanks made out of concrete will cause havoc in your water pH so they are not recommended, unless done and treated correctly.
The one easy option if going with concrete would be to either use a pond paint in multiple layers to seal the concrete away form the water or use a pond liner.
Fiber glass is often used and ideal for large outdoor operation as they last for a very long time.
Here you can find more about testing your Aquaponics Water pH.
The recurring and God Almighty question in the aquaponics world after what type of fish is: how many fish can I put in my tank?
The often given answer to this straight forward question goes like this: 0.5kg of fish per 25lt of water this gives you 20kg/1000lt, right? At this level of stocking it is mandatory to have active aeration, as in a well sized air pump with appropriate amount and sized air stones to deliver the correct amount of dissolved oxygen (DO). So with high stocking density (>25kg/m3) not only you will need aeration but you will also have to pass your total water volume through the biofilter (grow bed) multiple times per hour. For small backyard systems, this is not realistic as it would require large pumps moving large amount of water. Since we are in for growing plants and using the fish to help achieve this goal, there is no real need in using high stocking density numbers.
So, even with all what is discussed on the world wide web about aquaponics, I would suggest that you start your system with a stocking density closer to 10 or even 5kg/m3 (1m3=1000lt). When I say "start" I mean once it is cycled, of course! And even there, I would add the fish progressively, small changes at a time is key to success. See how to Cycle Your Aquaponics system.
Adding fish, is usually done with using fingerlings and so when we say 5kg/m3 we have to take into consideration, the final adult size of the fish, because this is how the stocking density is calculated. All the research around stocking density in aquaponics is done using tilapia and it is a fact that tilapia are harvested once they rich the marketable size of 0.500kg +/- plate size.
So, if we know this, now we can not only know how many KG/m3 we can stock but now, we can find out, how many fish does that represent: 5kg total and final harvest weight is 0.500kg so, 5kg/0.500kg= 10 fish for 1m3 tank. This is your answer of how many fish can I put in my tank.
How much fish feed?
How much feed per day will vary depending on your stocking density. So as we calculated above, we know how many fish and their total weight, what we will call, biomass. So, to calculate the feed rate, we use the total biomass. Fish eat a percentage of their body weight per day according to their age. So for sake of calculation, in this example I will use adult fish weight, just before harvest: 0.500kg/fish (10) and our total biomass in our tank is 5kg. The feed rate will be 1% of the body weight. So if our biomass is 5kg and we feed at 1% daily, we must feed our fish 50gms(*) of feed per day or 1.5kg per month or 18kg per year...
When putting these numbers in perspective, it is easy to see that aquaponics is not a cheap endeavor, specially in terms of input aka fish feed.
As a side note, fish feed comes in few variants: for omnivores or carnivores fish and the protein content will also vary between 32% and 45% respectively. The higher the protein content, the more the fish waste is produced. Keep this in mind.
Between grow bed size, fish tank size and feeding rate, all these numbers are declined from the study made in the University of Virgin Islands (UVI) with Dr Rakocy on a full scale aquaponics farm (See photo).
At the base of it all comes the aquaponics Golden Ratio. Very often on internet aquaponics literature, we can read that the ratio in aquaponics is between the fish tank volume and the grow bed area or its volume. However, this is not true since a more scientific approach gives the ratio to be: fish feed / growing surface area / day. The UVI aquaponics ratio gives us 60g/m2/day**
(*) 50g/m2/day is close to the UVI ratio considering a backyard system doesn't filter out solids
(**) Keep this number in mind, even if in high end of the feeding spectrum, it is useful to help determine the right grow bed area size. Keep in mind that this ratio is based on the UVI model and for us, backyardists, it is recommended to divide this number by 3 and have a baseline of 20g/m2/day maximum.
This is it for now ladies and gentlemen, the golden ratios as I see and understand them. Of course, you are free to tweak and experiment but I don't see the need to re-invent the wheel. It is also important to keep in mind that a media bed, even with red wigglers (earth worms) is affected by the solids and left over feed that accumulate over time. This is why, I personally fitted a dual external filter system where I do my best to separate the solids from the fish tank water. Plants do go on nitrate rich water, not on fish poop!
I hope you enjoyed this article and looking forward to reading your questions and comments.
I am a French expat, married, dad of one, nature lover and firm believer of sustainable living. My background is leisure and tourism industry. I fell into aquaponics and now I am sharing with you my adventure.