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Aquaponics 101 Part 6: the Ratio of Fish to Water

This is the sixth in a series of Tutorials that are going to teach you much of what you need to know about Aquaponics. So, if you’re curious about the most amazing food growing technology on the planet today, continue this series of educational Tutorials on Aquaponics 101 and please, become interactive by making comments or asking questions through our email address at urbanfarmer@aquaponicsusa.com.

In Part 1, “The Bio-Chemical 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). An AP system will allow you to grow food for you and your family year round as long as its in the proper environment. I also gave a description of the bio-chemical processes involved that make Aquaponics work.

In Parts 2 and 3, “The System Design”, 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 AP component called the grow bed, which is the most important part of an AP 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 Parts 4 and 5, “System Startup, Operation and Maintenance" I talked about an AP 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 AP system started and how to cycle your system.

I’m now going to focus on the Ratio of Fish to Water, which determines how many fish you can grow in your AP system. In this segment, I’m only going to discuss the most popular fish, Tilapia, among AP farmers because that’s the species with which I’ve had experience.

I'm going to tell you about an important item that needs to be addressed, the Ratio of Fish to Water. There are a number of fish to water ratios being given out by AP enthusiasts. So, let me start by giving you my preferred ratio of 6 gallons of system water for every pound of Tilapia as a maximum density of fish to water. As a beginning AP farmer, I recommend you stay at or above 6 gallons of water per pound of fish. But this is not the most important number in determining how many fish you can raise in a given system.

As the sun crosses the equator, we change the number of feedings per day. In the spring and summer, the days are longer supporting up to 4 feedings every three hours, as verses 3 feedings in the fall and 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 can also be less conducive to plant growth.

Another way to increase fish density is to remove the fish waste 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.

Through the process of solid waste mineralization, the nutrients and minerals found in the fish food makes their way into the plants. So, what you feed your fish is what you feed your plants.

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 system 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 about the system dynamics as it 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 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.

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Once the ratio of fish pounds to water gallons reached about one pound of fish for every four gallons of water we noticed the pH of the water dropping below the optimum of 7.3. As the fish continued to grow, the pH found its way down to as low as 6.0 ppm.

With the pH dropping below 7.0, the bacteria stopped properly functioning and the ammonia levels started to climb. Water exchanges were an immediate fix but not a long term solution. Adding in more bacteria and Potassium Hydroxide, pH Up, to the system brought the pH back up to 7.3 ppm and the ammonia levels soon went down to safe levels.

After about three plantings and many grown out flowering plants, the potassium had been mostly depleted from the system. The flowering plants stopped producing, leaves turned yellow and the cucumbers went from green to white. We now add Chelated Iron along with the potassium to keep our plant leaves and green veggies colorful and healthy. In some of the fish tank photos like the one below, you'll see that our water has a yellow tinge to it. That means we recently added the Chelated Iron, which turns the water yellow for a time.

An eleven square foot, deep-media (12″) 69 gallon grow bed holds about 36 gallons of actual expanded clay volume and 30 gallons of water. This includes about one inch of expanded clay above the highest water line to prevent algae growing on top of the expanded clay and mold growth on the leafy green plants bottom leaves. This gives about 60 gallons of wetted grow bed media being used as a bio-filter.

A two, 11 square foot deep-media grow bed system can support about 20 pounds of fish maximum with ample fish tank (120 gallons) and grow bed aeration, and works out to about 6 gallons of bio-filter/grow bed volume per pound of fish. Again, this one pound per six gallon of bio-filter number is a maximum and is only for a mature system, one that is at least one year old. Keep it safe and stay below one fish pound per six gallons of useable bio-filter.

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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, added aeration by way of air diffusers in the bottom of the grow bed does not reach all of the bacteria or vegetable roots as well as does draining and re-flooding.

As the fish grow out, they often 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.

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 again. 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.

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

Our breeder fish are always adding fry to the fish tanks, and as they grow out we will be pushing our system density. Every time fry are introduced into our system, the fish density in the system is being pushed, and we have had to keep an ever closer eye on the system chemistry. You need to find the balance between the fish you harvest to eat and the new fish coming into the system. If you don't eat enough, or any of your fish, the juveniles (who are carnivores) help you balance your system by eating the fry. If you separate the fry out from the rest then they will have a much better chance of survival, and at some point your fish numbers become just too great and your system becomes too unstable to continue to increase the density any further.

To summarize, increasing your fish density will allow you to produce more nitrates and other nutrients for your plants. As the fish density increases the system becomes less stable and requires more timely attention. This is true for a system of any size. As we found out, adding high pH water as a part of evaporation replacement worked well to keep the system pH in the proper range but as the fish density increased through time, the system started having new problems in the area of water chemistry. This required a greater and more timely attention and compensation in order to keep the system healthy. So, a six to ten gallon per pound ratio is a fairly safe and laid back place to be in a home or school aquaponics system. If you want to get the most from the least, then you can increase your fish poundage to water ratio. If you do that, then you will need to stay on top and ahead of your system, for it will go places it has not gone before.

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Please, let me explain.

These are the numbers we presently have in our system including our greenhouse, our enclosed growroom and our enclosed fishroom. Our system has a total of five 120 gallon fish tanks in that fishroom. Our Greenhouse has two, 11 square foot deep (12″) media grow beds, with a vertical duct growing system pictured above. Then there's a horizontal duct growing system in the growroom.

Except for the two deep media beds, this is not a normal backyard or school system. We are running what we call a Micro Food Forever™ Farm, which is our Proof of Concept for our larger Food Forever™ Farms. If you want to learn more about our Food Forever™ Farms, click on the link. Nevertheless, we are holding almost the same ratio of fish to water as we used when we had only five deep media beds and three 120 gallon fish tanks in our Greenhouse alone.

We recirculate the water between the five fish tanks so they act like a 600 gallon fish tank in their biological stability; and the greenhouse and the growroom share the same water. What we found as the fish grew out and are now averaging about 2 pounds each, is that the 55 square feet of 12 inch deep media grow beds adequately converted 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 actually has a total of about 500 gallons of water in it and about 100 pounds of Tilapia for a gallons of water to pounds of fish ratio of about 5:1.

With an increased amount of bio-filtration (more grow beds), it may be possible to decrease the water to fish ratio to a lower 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. 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.

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 feeders we use have an eight event per day timer and can be set to dispense food in one second intervals and are available on this web site. By setting the timer to dispense food for ten seconds, three to four times every day, which is about every three to four 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 for a few days 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 using an automatic fish feeder.

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Congratulations! You've just completed Aquaponics 101, Part 6.

Now it's time to test your knowledge. Take the Part 6 Quiz here:

1.  What is my recommended of water to fish ratio?

2.  If you increase the area of bio-filtration (grow beds), it may be

    possible to decrease or increase (Circle one) the number of

    gallons of water per pound of fish in the fish tank.

3.  What is a way to safely increase fish density?

4.  The real fish to water ratio has to do with the amount of fish

    food digested and the size of the bio-filters needed to process

    the waste.

    True or False (Circle one).

5.  What is the easiest and most efficient way to feed your fish?

6.  What is a 3rd way to increase fish density?

7.  When the pH drops below 7.0, the ________ stop functioning

    properly and the ammonia levels climb.

8.  What do you need to add to your AP System to keep your

    plants green instead of yellowing or going pale?

9.  What's the minimum age of a system that's considered to be

    mature?

10. You always need to plan ahead in your system design regarding

    the total fully grown poundage of ______ in your system.

11. An AP System is designed for either maximum _______

    production or maximum _______ production, but not both.

12. How do the fingerlings and juveniles in your AP System help you

    keep your system in balance?

13. As the ______ density increases, the system becomes less

    stable and requires more time and attention.