Aquaponics 101 Part Five: Fish to Water Ratios

This is the fifth in a series of posts that are going to teach you much of what you need to know about Aquaponics. These posts are part of a book we are writing and will be selling on our website. So, if you’re curious about the most amazing food growing technology on the planet today, watch for this series of educational posts on Aquaponics and please, become interactive by making comments or asking questions. Given the state of our Union, the motto on our website is becoming more important than ever–”Time To Grow Food”.

In Part One, “The Process”, I wrote about what Aquaponics is and why it is important to Preppers (those preparing for what is about to come down the pike), the fact that you can grow food for you and your family year round as long as your Aquaponics system is in the proper environment. I also gave a description of the biological processes involved that make Aquaponics work.

In Parts Two and Three, “The System”, I wrote about the components of a basic system. To quickly review, I wrote about the need for a bio-filter and that it is usually combined with the grow bed to form a single Aquaponics component called the grow bed, which is the most important part of an Aquaponics system. I told you about the grow bed media, the grow bed shape, and that you need about one gallon of grow bed/bio filter volume for every gallon of fish tank volume and the reason for this ratio. I discussed the need to flood and drain your grow beds four times an hour and how to properly size your water and air pumps.

In Part Four, “System Startup”, I talked about an Aquaponics system water and all the important aspects of the water like the DO (dissolved oxygen), nitrites, nitrates, pH and alkalinity. I talked about how to measure the water using a freshwater test kit, TDS meter, a DO meter and a pH meter to determine that it’s safe for the fish and the plants. I also explained how to get an Aquaponics system started and that it needs to be “cycled” before fish or plants are introduced.

I’m now going to focus on the fish in an Aquaponics system; but I’m only going to discuss the most popular fish, Tilapia, among Aquaponics farmers because that’s the species with which I’ve had experience.

Before I go into the nature and breeding of Tilapia, I need to talk about another important measurement that needs to be addressed, the ratio of fish to water. There are a number of fish to water ratios being given out by Aquaponics enthusiasts. So, let me start by giving you the ratio of 3 gallons of system water for every pound of Tilapia. But this is not the most important number in determining how many fish you can raise in a given system. Please, let me explain.

This is the number we have in our systems in the Aquaponics USA greenhouse. Our systems have a total of three 120 gallon fish tanks and a total of five 10 square feet of deep (12″) media grow beds, as we didn’t have room for a 6th grow bed to balance it out.

We recirculate the water between the three fish tanks so it acts like a 360 gallon fish tank in its biological stability. What we found as the fish grew out and are now between 1.5 and 2.5 pounds each, is that the 50 square feet of 12 inch deep media grow beds adequately convert the fish waste (including solids – without accumulation), into plant nutrients. We add heterotrophic bacteria weekly to help with the solid waste mineralization and for water clarity.

Our current system has a total of about 300 gallons of water in it and about 100 pounds of Tilapia for a gallons of water to pounds of fish ratio of about 3:1.

With an increased amount of bio-filtration (more grow beds), the water to fish ratio can be decreased to as little as 2 gallons of water per pound of fish. With a decrease of bio-filter volume then this ratio must be increased. This is based on feeding the fish as much as they will regularly eat and as often as is practical with no food left in the tank.

Another way to increase the fish density is to decrease the amount of food given to the fish. This will slow their growth, which may be desirable once they are fully grown. In any event, the real ratio here has to do with the amount of food digested and the size of the bio-filters needed to process the waste.

We have found that the best way to regulate and insure that the fish are fed portioned amounts of food and on schedule is to have an automatic fish feeder. The ones we use have an eight event per day timer and can be set to dispense food in one second intervals. By setting the timer to dispense food for ten seconds, three to four times every day which is about every three hours, we know that the food they receive is the same every day. This allows us to adjust the amount of food given the fish as needed, and it also allows us to leave without worry that they won’t be fed. As the fish grow out, we increase the number of seconds on each feeding, thereby giving the fish more food. We highly recommend the automatic fish feeder.

As can be seen, the pounds of fish the system can support is more a function of the amount of bio-filter volume available than it is to fish tank size. This assumes proper design and selection of other system components.

In the summer, the days are longer supporting up to 4 feedings every three hours, as verses 3 feedings in the winter. This allows for more food and, therefore, waste in the system. Couple this with warmer water temperature, and we have a faster fish growth rate as well as more nutrients available for the plants. The warmer water also contains less dissolved oxygen (DO), which is problematic. The warmer water is also less conducive to plant growth.

Another way to increase fish density is to remove the fish solids from the system. This would un-tax the system from the need of some of the DO (dissolved oxygen) in the water and would reduce the amount of system ammonia; but it would also reduce some of the resulting nitrates. In addition, this would remove some valuable plant nutrients that are a result of the mineralization process of fish solid waste. So, we leave in the fish solids, add heterotrophic bacteria weekly (see Aquaponics 101 Part Four: System Start Up) and provide ample aeration.

The key to this all working is adequate DO (dissolved oxygen, see Aquaponics 101 Part Four: System Start Up) in the water. As we measure it weekly, we have changed our design over time to improve this important factor. Because the fish waste conversion to plant nutrients requires ample DO, we have a DO meter; and in using it regularly, along with other water chemistry measurements, we learned a lot about the system dynamics as the system matured.

As our fish grew out and we increased the amount of food we gave them, the DO in the water decreased over time, which affected the nitrification process. DO is also a function of water temperature and this must be accounted for in making DO measurements. We then added aerators to our grow beds and also increased the aeration in the fish tanks. The DO then increased to a good level for the mineralization of the fish waste solids and the nitrification of the ammonia in the system.

Two ten square foot, deep-media (12″) grow beds hold about 66 gallons of Hydroton each for a total of 132 gallons. This includes about one inch of Hydroton above the highest water line to prevent algae growing on top of the Hydroton and mold growth on the leafy green plants bottom leaves.

A two, deep-media grow bed system can support about 40 pounds of fish with our current design of ample fish tank and grow bed aeration, and works out to about 3.3 gallons of bio-filter grow bed media (Hydroton) per pound of fish.

Using the above ratio of 3.3 gallons of grow bed media per pound of fish we then get the following:

Grow Bed ft.….Fish Tank Gallons…Media Gallons…Pounds of Fish
2X10′ Beds……..……………120…………………132………..……….40
4X10′ Beds……….………….320…………………264………..……….80
5X10′ Beds……….………….320…………………330……….………100
8X10′ Beds…….…………….610…………………528……….………160
10X10′ Beds…..………….…610…………………660……….………200

It is important that the grow beds all be of the flood and drain (ebb and flow) design. This allows for the bacteria on the surface of the Hydroton and vegetable roots to be aerated between the drain and flood cycles. Even with added grow bed aeration, which adds additional DO to the water, it does not reach all of the bacteria or vegetable roots as well as does draining and re-flooding.

As I mentioned near the top of this post, you can reduce the gallons of water per pound of fish to as little as 2:1 by adding more grow beds/bio-filters. The problem with this is, if all the grow beds are flood and drain, then at some point in a set of auto-siphon grow beds, they will all drain at the same time filling the fish tank to capacity. The grow beds will then all begin filling simultaneously and the level of water in the fish tank will go dangerously low. This can be avoided by either not having all the grow beds flood and drain, thereby limiting them to growing green leafy veggies and not doing as good of job as a bio-filter; or by using a rotating timing system to fill each grow bed in turn and letting it drain on it’s own, which adds complexity and additional failure points to the system.

A system could also be designed with an additional sump tank to absorb the variation of fish tank water height. But you might as well go for a larger fish tank and save the cost of an additional tank and perhaps an added pump. We have looked at all these possibilities, priced out the various configurations and determined that the design I’m describing here is the most economical as well as the simplest.

You will notice that I always mention Hydroton for the grow bed media, as these systems were designed and tested using it. This is not to say that other types of media, such as river gravel, won’t work in these systems; but you will not get the same surface area as you do with Hydroton; and, therefore, you may see a decrease in bacteria growth and activity. This will require you to reduce the amount of fish poundage in the above table. Gravel is also more difficult to work with than is Hydroton as well as being heavier.

Gravel tends to occupy more space than does Hydroton in the grow bed, which means less water and therefore less exchange of water to the fish tank during syphoning. This reduced water exchange will allow for additional grow bed/bio-filter space to be added and will require it to produce the same filtering as the beds using Hydroton.

Adding more horizontal grow bed space allows for more planting area. While at first this may seem like a good idea, you can easily over plant your system and pull out more nitrates than desired thereby starving some plants from needed nutrients. This is especially true if you plant flowering plants like tomatoes. It is good to always have some grow beds ready to be used as you rotate the timing on your planting in order to have a continuous supply of fresh veggies without over taxing your system with too many plants.

The pounds of fish column in the above table is a maximum total in the system. As the fish grow out, they do so at different rates and you need to plan ahead for the maximum size they will be before harvesting. If you do not plan on harvesting your fish and choose to keep them in the tank for their life, then you need to plan on the maximum poundage of each fish when fully grown. Breeding your fish for sustainability is another option you may want to consider. We will be doing another post on breeding Tilapia so be sure to watch for that one.

It is important to note the difference between maximum fish poundage and yearly pound production. Tilapia (as with most fish species) tend to grow faster in warmer water with longer daylight hours, natural or artificial. It is possible with certain Tilapia species to grow to market size (1.25 pounds) within 6 months. This would give you two crops of fish per year in a single tank thereby doubling the maximum poundage numbers to get annual yield poundage.

This brings up the concept of system design. A system is designed for either maximum fish growth or maximum vegetable growth; but not both. Keeping your fish water warm and removing fish solids is a method of maximizing fish growth and are some of the techniques used in aquaculture, as is adding pure oxygen to the water. However, warm water is often not conducive for good vegetable growth. These and other factors determine what it is that you will accomplish in your system.

As always, monitoring your water quality regularly and adjusting the amount of feed given the fish to maintain that quality is important.

Best Regards
Oliver Duffy

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13 comments on “Aquaponics 101 Part Five: Fish to Water Ratios

    • We have some LED’s and they worked okay once we got the right ones. We’ll be reinstalling them once we re-design this growroom. I’ll be doing a post on that when it happens. Thanks for the feedback and following us.

  1. I run a water garden department at a garden center. Can I use Koi an Comets instead of Tilapia? I can get any size fish, within reason, in any quantity.

  2. Hi, I am from Equatorial Malaysia where lifeforms of diverse varieties thrive without even trying. I am convinced Aquaponics is the way to go to achieve personal food security via integrated farming. I am considering powering the circulation pump via Solar Photovoltaic given the more than 360 days of sunshine days x ESH of 4.5hours per day in Malaysia. I am a novice to Aquaponics and your well written and greatly informative Aquaponics 101 has already given me the quick jumpstart information necessary to plunge into Aquaponics by shortening the learning curve and avoiding all the avoidable common pitfalls. Cheers and Thank You!

  3. I think you are going about it all wrong.
    First during start-up phase you should not put all your intended fish stock in the tub. Only enough for a segmented harvest. Add more at each interval to you wish to harvest. I suggest making this in weekly or biweekly intervals. Until all of your tanks in the system are full of fish. You need to do this the same for the plants. The affect of this is that on a weekly basis it keeps all the chemicals in the system constant, thus adding allowing for less stress on worrying about DO, ammonia and nitrates. This optimally would require a reservoir inflow and drain tank, due to the necessary compartmentalization of the fish tanks and keeping the inflow chemical ratios drain chemical ratios constant by allowing them to mix in the reservoir.

    Also you should measure the weight of the fish at each stages of their cycle of life, during that week during the seasonal cycle. That’s roughly 52×34=1768 permutations. It should take roughly 3 years to catalog initial findings, another 6 to find a reliable mean.

    From a efficiency aspect and a business aspect this is the only solution viable. Plants, fish and business partners don’t like change. This minimizes changes in variables and allows you to standardize the workflow to allow easy training of employees as well as its impossible to build an effective business around only selling fish only 1 time every 7 and a half months.

    I hope this helps you.

    • Dear Chrisopher:

      This is great advice for larger commercial-type operations; and we’ll be staggering and staging fish and planting in our commercial operation. But for home and school systems, they are so small, we believe they need their fish right off the bat.Thanks so much for your suggestions.

  4. I have seriously been considering aquaponics. Your site has been very informitive. Instead of jumping into it I will go slower. This will allow me to gain experience and confidence to grow more products.
    Thank you

  5. I have started my aquaponics system already, I would just like to know if a 6ft x 3ft x 1ft grow bed is large enough (or to small) for a 100 gallon tank and 25 fish?

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