Frequently Asked Questions (FAQ)


1) What is the maximum production I can expect from the 4 -6’ x 24’ tanks for tilapia with O2 injection?

A   The maximum bio-mass (total weight of all of the fish in a tank at one time) in a 3500 gallon tank would be about 2500 pounds.  The total annual production would vary according to the species you are raising and the size of the fish to market. For instance, tilapia can go from a 30 grams fingerling to one pound in 6 months. If your market fish is sold at one pound then you can get two crops per year. If at the time of market there are 2500 one pound fish in the tanks each cycle then this would work out to about 5,000 pounds per year per production tank.  However, there is a veritable here as well. I recommend 4 tanks to start because in reality only three of these tanks are going to reach capacity each year. This is due to the fact that fish must be graded at least once a month as a major part of the management program. Not all fish grow at the same speed. Some of the population will be very aggressive and grow very fast. These animals, about 25% of them,  must be separated from the rest of the fish. Other wise they will dominate the tank and keep the more passive ones from getting enough feed. They will also bully and stress the passive ones.

The way the management plan works is like this. To begin with never think of a loaded tank by fish numbers, always think bio-mass.  When you first start up the system and make your fingerling purchase you have to decide on how many to get. If you are going to run four tanks you technically need 10,000 fingerlings. For practical purposes you should get 15,000. This is because you will have some mortality plus, remember, some of these fish are going to grow very fast and go to market in five months while others are going to take up to 7 months to grow.  When the fingerlings first arrive place them in equal numbers in just two tanks.  After a month you will notice how fast some have grown. Now you are going to use the other two tanks. Fill them and get them running with water of the same temperature and quality as the first two. You will then go to one of the first tanks and grade them out in two different sizes ranges. The larger one in one tank, the smaller ones in another tank.  Repeat this process with the second tank.

A month later, assuming you have had god luck with low mortality you will grade these fish again. Only this time you will see a wider difference in sizes and will spread them out into three tanks.. By the end of the forth month you will have one tank that contains as many as 2,000 to 2500 of the faster growing fish. This is called the market tanks. The other tanks will have smaller sizes but they too within a month will be getting large enough to make it to the market tank. Eventually about 85% of all the fish will get to the market range. Keep in mind it takes almost a year to get everything moving in a good rotation.  New fingerlings are purchased to replace the numbers sent to market.

2) What is the expected tomato production?

Experts in the aquaponics fields show a range of 15:1 to 25:1 ration between the amount of fish you grow and how much veggies you can grow. In other words, for each 1 pound of fish you can get between 15 pounds and 25 pounds of veggies in return. This is not just for tomatoes, but for all of the different crops you can grow like lettuce,  herbs, peppers etc..

3) What is the cost of fish food?

You can still get good fish food for about 30 to 35 cents per pound. Cargil Nutriena is a good food. Up[ until about 1/4 pound the fish should get feed that is 35% protein. After that it can be cut back to around 28%.

4) Can I mount the tanks in ground to reduce the energy costs in the winter months? ( I realize you have the elevations set up for gravity flow)

For easy management it is best that the tanks be above ground, usually set at about 36″ above. It is very difficult to work the tanks when you have to bend over all the time.

5) What do you think about duckweed as a food source for tilapia?

The only way a farmer might reduce purchased feed costs is when he is growing a carnivore and he grows minnows or something like that. But even then he has cost to do that.  Thus far, the purchase of high quality formulated feeds has proven to be the most economical. The largest fish farms in the world still purchase manufactured feed.

About the duckweed,

Here is a website to explain some things

Ok, Here is the problem with duck weed. Let’s first look at what is says in the page Actual yields of fresh material from commercial-scale cultivation of Spirodela, Lemna, and Wolffia species at the Mirzapur experimental site in Bangladesh range from 0.5 to 1.5 metric tons/ha/day, which is equivalent to 13 to 38 metric tons/ha/year of solid material.

Nutritional value Fresh duckweed fronds contain 92 to 94 percent water. Fiber and ash content is higher and protein content lower in duckweed colonies with slow growth. The solid fraction of a wild colony of duckweed growing on nutrient-poor water typically ranges from 15 to 25 percent protein and from 15 to 30 percent fiber. Duckweed grown under ideal conditions and harvested regularly will have a fiber content of 5 to 15 percent and a protein content of 35 to 45 percent,

What this says is, in this experimental station they get from .5 tons per 2.5 acres per day to 1.5 tons per 2.5 acres a day (1 hector). We experimented with it many years ago in the middle of summer when growing conditions were perfect and I am leaning more to the lesser amount and I even question that. To narrow it down more, they are using 2.5 acres (1 hector) to get 1,000 pounds of duckweed per day.  Now look at the nutritional value. Duckweed is 92% water.  So when dried, because that is the only real feed, you are left with 80 pounds of 35% protein (The same you get with store bought feed) . So here is what you have. 2.5 acres of ponds dedicated to this. Labor of harvesting and then drying to at least 50%,  because, you do not want your fish filling up with water.    In order to feed one of our fully loaded 5 tank system you need at least 210 pounds of feed per day. So that means you will need 7.5 acres per day to feed your fish and they must be maintained to perfect temperatures and nutrient levels.

In addition to this, while duck weed might have the protein, it does not have the trace minerals and vitamins found in commercial feeds. This is why beef farmers grow the cattle in feed lots. They get faster growth from commercial feed than they do if they just let the cattle graze out in the pastures. Given commercial feeds in a tank system, a tilapia will grow from 20 grams to 1 pound or more in 6 months. Given natural feeds without the trace minerals and vitamins it could take up to nine months to reach 1 pound.

Now, in a tropical third world country where they do not have access to commercial feeds, duckweed is a good alternative. But you are in a country where time is money. The longer that fish is in your tank the more he is going to cost you.

6) How much make up water is required?

This varies from just few gallons per day to certain times when a tank must be emptied half way and new water put in the tank. I would say on an average over year  figure about 50 gallons per day per tank.

7) Do you think copper tubing would be better for heat transfer in the heater sump?

Never ever use any copper anywhere in a fish system. The copper reacts with the fish water and makes copper oxide and other bad things that will kill the fish

8) What does the digester do?

The digester is the first stage of the aquaponics system. This iswhere all of the fish waste leaves the aquaculture part.  There are three stages to getting the fish waste in a form that the plants can use. All of the fish waste from flushing the bottom of the tanks each day goes here. All of the solids collected in the clarifier go here and the solids collected in the biofilter go here after flushing. In the first compartment the waste is broken down just like in a septic tank (anaerobic) . By gravity as more waste is added the liquid flows to the second compartment. Here there is a circuator pump that just rotates the liquid and splashes it to add air to the fluid. (Aerobic). By the time the fluid reaches the third compartment it has digested down to a rich nutrient that the plants can use.

9) What roll does the plate clarifier do? How do you clean the sludge? What do you do with it?

The water from the fish tanks is constantly circulating from the tanks, through the plate clarifier and then to the biofilter and then back to the tanks.  The plate filter removes all of the solid waste from the water before it goes to the biofilter.  The biofilter is not really a filter as we know it but is a bacterial contactor. Here bacteria in the filter convert the ammonia and nitrites to nitrates.

One a month the system is completely shut down and a submersible pump is set down in the bottom sump of plate filter and all of the water is pumped over to the first compartment of the digester tank. When the water is all out each of the filter screens is removed and sprayed down with water to remove all of the trapped waste material. This is sent over to the digester also. After the plate filter is clean reload it with water from the fish tanks and restart the system. While this filter is being cleaned the secondary injector pump and injectors are running to keep the oxygen level in the tanks up.

Is there a reason the plate clarifier is topless?  Rather than have an open pit we are thinking of covering it with a grate you can walk over.

The plate filter is designed to be open so that it’s plates can be observed on a frequent basis. When operating the water should start out flowing eveningly through each filter plate because there is nothing to clog the plate screens.

After a few days you will see that the water is higher in front of the first screen and is flowing over the top of the screen. This is a good thing. Up to a point, the more the screen clogs the smaller the particulate the screen will capture. This is why there are solid plates in front of and behind the screen. It puts some light pressure to push the water through the screen. In time the same thing will happen to the next screen and the next and so on. I recommend that after you see three or four of the screens over flowing it is time to clean the filter.

I do recommend that either a grate be placed over the filter box or a chain rail and post type guard be placed around the box to keep someone from stepping in the filter.

  1. Does the clean out sump at the end of the plate clarifier extend to the opposite un viewed side in the drawing or exist only in the front side?

The sump is the full width of the box. This is used for cleaning the box and also for allowing some suspended particulate to settle out before if heads to the bio filter.

To clean the box you do the following.

First manually turn on the secondary injector pumps if they are not already on. This will keep the circulation going in the tank and also keep the oxygen flowing.

Then shut off the pump from the biofilters to the tanks.

Then shut down the pump from the plate filter to the biofilter.

Within a few minutes all water from the tanks will stop flowing to the plate filter.

You will need a spare ½ hp submersible pump with a hose. Drop the pump down into the sump portion of the plate filter and place the hose in the first compartment of the digester if you are using aquaponics. If no aquaponics are in use then this waste must go to your waste tank. This is very bad stuff so never allow it to go back to the fish system.

As the pump is draining the plate filter box remove each screen from the box and wash it out fairly clean allowing the water to fall into the filter pit. It is not necessary to get this screen sparkling new clean. There is some bacteria in there that acts as a part of the bio filtration, so a little grime is a good thing.  Lay the screens on the floor until the entire process is complete. Spray down the side walls and bottom of filter pit.

Remove the pump and replace the screens with clean ones.

To restart chose any tank you wish, usually the one you consider to be most in need of flushing start adding water back at the injector end. This will cause the water I nthe tank to go through the over flow and refill the pip filter.

Once the pit filter is full turn on the pump to from the filter to the biofilter. As soon as water begins to exit the bio filter turn the injector pumps on and the system will once again be in operation.

NOTE:  Never fill the pit filter with clean water. This water will not have any ammonia or nitrite in it and could cause a starvation of the bio filter.

  1. What weight fiberglass cloth is recommended to line these boxes with?

A light 3oz. cloth is plenty. This is just something to protect the wood and provide a solid seal so it doesn’t leak.

  1. I note an oxygen flow meter and an air compressor feeding air into the injector line, but not an oxygen generator as I think I see on page 15 of your main web site.  Is plain air and not oxygen injected into the water?

There are two ways to get an oxygen supply in your facility. One is to use ambient air (compressor) and the other is to use pure oxygen either from an oxygen generator or to purchase it from a local supplier (liquid oxygen).

Shown in the drawings is the least expensive way to get the oxygen by using a simple air compressor. This will do a good job but unfortunately you will grow less fish per gallon of water because this in this source on 20% of the air is oxygen.  It matters not which source you use the air must be passed through the oxygen flow meter to regulate the amount you are putting in the water.  For pure oxygen you put in about 4 liters per minute. For ambient you put in about 8 liters per minute. This is something that takes a little experimenting with by the operator to decide what works best in your system. If you put too much gas in the water for the water flow the injector will start bubbling straight up at the discharge point. The flow must be turned back to the point where the heavy bubbling stops and you only see small bubbles coming out a short distance from the injector discharge point.

  1. Is it feasible to integrate the computer onto a 10 tank system?  We are thinking of 8 production tanks and two fingerling breeding tanks and would be interested in the added “insurance of computer monitoring.

Using computer monitoring and controls is always a good idea, even with just one tank. Once again I did not include this into the drawings because of the added cost. There is a very good system located at  I have a design on a monitoring station that you can build yourself but will have to do some new drawings later this year to up date it to be compatible with these newer electronics. When using computer monitors the secondary pump in your tank plays a greater role for oxygen control. When using pure oxygen most of the time the main injector will supply all the oxygen you need for the fish. At times, such as feeding when the oxygen drops the controller will turn on the secondary pump and injector to add supplementary oxygen to the tank and turn off when it reaches a pre set level.

On sheet 2 there are water storage tanks and an air compressor shown in the furnace room.  I would like details on these two items for purchase or construction.

Once again in order to keep cost down I have not shown any particular specifications for these tanks. Some have used the same type of tank shown for the smaller bio filter. These are actually totes that companies get some of them chemicals in.  Two places I know  of that get things in these tanks and them throw them away are water treat facilities and concrete companies. These can be any fiberglass tank. Other places you can purchase them are any agriculture supply company and perhaps even Tractor Supply. I recommend that their total capacity not be less than 500 gallons. This is where the new water for the system is heated to operating temps and then discharged to the tanks when needed. It is a good idea to place a ball cock on these tanks to keep them always filled.

The air compressor should be in this system even if you are using pure oxygen and should be a minimum of 5 hp . This is critical if there is a disruption in your oxygen supply. If not using pure oxygen I would recommend that a back up compressor also be available in case of primary failure.  Keep the tank pressure on the compressor at 150psi and use a regulator to bring it out of the compressor at no more than 20 psi.

What are the specs on the internal circulation pump on sheet 5?

These pumps can be up to 1 hp, depending on what you can find at a bargain. The minimum size is ½ hp.

3. In the plate filter design on sheet 8 the overflow pipe is shown coming in at the clean out sump end.  On the total system drawings it is shown entering at the opposite end – where an empty unidentified hole is present on sheet 8?  Also the hole in the side at a right angle to the overflow pipe is unidentified – but appears on sheet 12 to connect a pipe thru a valve pit into compartment 1 of the digester tanks?  Sheets 2, 8 and 11 show no connection between digester tanks and the plate filter.  I’m confused.

4. How does water dissipate out of the digester tank other than thru the aquaponic system?  I see it entering thru the tank waste lines, the biofilter flush line, and the return line from the aquaponics system (plus from the plate filter if the connection shown on sheet 12 is correct), but the only exit is back into the aquaponics system.  Seems like it would get overrun so I must be missing something.

5. Is the 1/3 hp submersible agitator pump detailed on sheet 10 used in the middle tank of the 3 – 1000 gal concrete septic tank system shown on sheet 11?  I don’t see it mentioned or a power source into the tank for it.

6. Where can one obtain the aquaponic plant troughs, or can they be constructed of marine plywood and pitch like all the sump pits?

7. Water is force pumped out of the digester tanks into the aquaponics system, but the return appears to be all gravity feed but on a level run.  Is this correct?

8. On sheet 14 it says to refer to the specs on the bio media but I’m having trouble locating those specs.

9. Well we be getting instructions on things like (1) how often to open clean out valves from bio filter back to digester tanks (2) how often to clean out plate filter screens/tanks etc.

10. On sheet 2 the balancing line between the heater sump and the plate filter sump calls for 6″ pvc, where on sheet 16 it calls for 4″ pvc.  Which is correct or preferred?

11. Do your systems meet EPA requirements for fish containment to be sure they can not escape into the wild?

Just to give you some history, I am located in one of the most environmentally critical areas of the country. I live just 6 miles from the Chesapeake Bay in Maryland. The regulations for animal waste runoff is more closely scrutinized here by both the State and Federal Government than anywhere else in the country.

As a result, all of my research and development of my system designs have had to meet these guidelines. It was because of these concerns back in the 1980’s we started developing the aquaponics to keep from having discharge from the systems.

In my system designs all of the waste from the fish tanks go to aerobic digesters to digest the solids. The results is a highly enriched nutrient product which is perfect for the growing of plant crops. These digesters can be set up to serve two purposes at the same time. One of course is to provide food for the aquaponic crops. The second is the drains fields can be set up much like a house hold septic system and the excess water can be discharged this way. I am not a big fan of doing this because there is still nutrient in the water. However, the best way to get rid of the excess water is by applying it to crop fields. Because it is already digested it is ready for plant or grass consumption unlike other animal waste that has to lay one the ground for a while so it can breakdown. In the state of Maryland and most other states, fish waste is viewed like any other animal waste so there is no special certification here. There is little difference between fish waste and that of a cow except that it is applied as a liquid instead of a solid. Most cattle farms in the country these days send the cow manure to a waste pit to allow it to turn to liquid first.

In a fish system you can expect to discharge about 3% of the water per day through filtration. As mentioned, in my designs the water goes to the digester holding tanks. If you have a 5 tank 25,000 system and 12,000 square feet of plant crops growing, they are going to use up every bit of this water and then some.

There is no extra discharge during any tank changes. All systems have what we call buffer tanks in them to save the water for reloading. It is not unsual for operators to drain some of the fish tank water to the aquaponic digesters to make up for the plants usage and then add fresh water to the fish tank.

One of the biggest myths there is by critics of the industry is the “Creation” of pathogens in a fish system. Any pathogen, virus or other fish disease found in a fish tank came in from the wild. In other words, if you get some disease in your tank you can just bet it came from someone bringing it from outside local sources.

Back in the 1980’s here in Maryland Columnaris  was found in our fish tanks. One particular critic of the industry, who had a special interest political agenda, made a big deal out of it. He actually claimed that because we were growing these fish indoors we have “Created” a new kind of disease and that the wild fish would be destroyed. A study of the Chesapeake Bay later that year found that the pathogen was wide spread throughout the Bay. It seems that prior to this aquaculture incident they had just never looked for it before. Therefore, for anyone to think that a fish farm can somehow “Create” a pathogen or disease that does not already exist, they are giving “God” like powers of creation to the fish farmer.

There are many steps to go through in a fish farming operation to keep the disease occurrences down. There are laws that require all hatcheries to have their fingerlings certified as being disease free before they can be shipped across state lines. Most fish farms have bio-security plans in effect to keep the visitors to a minimum and to disinfect the workers each time they enter the building. This is why many fish farms will not allow visitors.

One of the purposes of a modern indoor, Zero Discharge, fish farm is to be certain that no animal can escape the system. They can not. The only way a fish can leave this system is to be transported out by someone. There is no active stream from the tanks to an outside water source.

The labor depends on the type of system you build. In the more simple S-2005 system you will have more work to do each day and could take one person an hour or two to clean the plate filters. In the S-09 using the drum filter there is no cleaning. Aside from feeding and some little daily maintenance there is very little to do. One person can run a 10 tank system all by themselves. There are times of grading and such that having a helper would be nice.

The aquaponics are different manner. Tending and harvesting plant crops can be labor intensive. I would say a single 3,000 sq ft green house could keep one person busy all day long.