High Density Systems

This is a looong post; we couldn’t figure out any sensible way to divide it into separate posts. It covers High Density aquaponics systems, which are systems that are designed to raise a maximum amount of fish.

This seems like an “of course”, unless, of course, you’ve realized you’re losing money on the fish (we lost $12,000 on the fish our first year in aquaponics). We don’t have a trust fund, nor do we have our expenses paid like the university that first developed High Density aquaponics systems, so we use Low Density (LD) systems (also covered in this blog) that lose a minimum amount of money on the fish; they grow exactly the same amount of vegetables as HD systems AND make a profit overall. So this is NOT a recommendation that you build one of these systems, simply an explanation of how they’re put together.

(Below) Our first HD system: fish tank, solids settling, net tank, and degas tank (combined into two tanks with our fiberglass/plywood/epoxy tank technology).

IMPORTANT: Our farm’s first three systems were originally stocked with fish at a level of 1.5 to 2 lbs. of fish per square foot of raft area. We call a system with this stocking density a High Density system (HD for short) because of the high density of fish they support. With the high fish food, electricity, and labor costs we have in Hawaii, these were not the best systems to use because we lost $2 on each and every pound of fish we grew (LOTS more on this in the post Growing Lots Of Fish!). When we figured out our LD systems, we converted these three HD systems into LD systems and lost a LOT less money on the fish portion of the operation.

This section will explain the components in one of these HD systems, but PAY ATTENTION! We are NOT recommending that you build one unless you have a very specific economic climate at your location (or a trust fund, LOL!). If you have any questions or confusion about this, you should build an LD system! You can always turn it into an HD system later by adding a few thousand dollars worth of tanks and plumbing if you find that you are making money on the fish. A decision to build an HD system rather than an LD system should be based on the economics of raising fish at your location: the costs for fish food, electricity for aeration, and labor need to add up to a lot less than the wholesale price you get for your fish, or you are going to lose money on the fish!

What Do We Recommend For You: Low Density Or High Density System?

A student once sent us this question: “I presume you recommend LD stocking for commercial systems including the 4096 sq ft size?”

Our answer: We don’t recommend any kind of stocking for our commercial systems: we recommend you read the section on LD and HD systems in the manual again, and compare fish food, labor, and electricity prices with wholesale fish prices in your location; then make a reasoned business decision in favor of profitability. We don’t know those numbers for your location, thus, any recommendation from us would just be a guess.

Here’s the “back of the napkin” formula to figure if your fish will be profitable: It takes about 2 lbs of food, about 3 KwHours of electricity, and about 0.11 hour of labor to raise 1 lb of tilapia to maturity over the lifetime of the fish; and to harvest the fish.

At our location, with our costs, this equation is: (2 lbs of food X $1/lb) + (3 KwHr X $0.44/KwHr) + (0.11 hour labor X $10/hr) = $4.42 per pound. If it costs us $4.42 a pound to raise a fish, we’d better be getting $5-6.00/lb wholesale for the fish, or we are losing money on the fish portion of the operation. Unfortunately, in our location, the wholesale price of fish is $2.50/pound!

Put in numbers for fish food cost, electricity cost per kilowatthour, and labor cost per hour for your location, and you will have a rough yardstick to determine whether your fish will make you a profit or cost you money. If it looks like a very small profit, breakeven, or a loss, stick with the LD system; you can always modify it later to be an HD system!

If you are caught by the glamour of raising a lot of fish, or just buy some salesman’s pitch about raising fish and don’t check into the costs of doing so first, please don’t get upset with us if you lose money doing so. We tried to warn you.

Our HD/Commercial systems have eight major components: a fish tank, a solids settling tank, a fine solids capture tank, a degas tank, vegetable troughs, a sump tank, a water pump, and an air pump or blower. They are more complex and expensive to build and operate than LD systems, and grow no more vegetables than our lower-cost LD systems do per square foot of raft area. They, like our LD systems, are apparently bulletproof: we started operating our first such system at the beginning of a long cold period. It was cold and gray for three months, and we only had 100 pounds of fish in a system we “knew” needed 1,200. Even with these problems, we had the same vegetable production from this system in these minimal conditions at the beginning as we got later when it was mature and full of fish (1,200 pounds). Here are the components and sizing for three different HD-type commercial systems that we operate, that shows you the viable range of system proportions we’ve experienced:

System Name                          System #1                   System (#2)               System #3                       

We redesigned the troughs between building system #1 and building system #2 to be 8″ deep water instead of the 12″ in system #1. There was no detectable difference in vegetable growth, water temperature in the systems, or system operation due to this 33% reduction in depth and water volume in the troughs, and it saved us a ton of money on the trough construction.

The flow direction of these systems is always the same, because water always flows downhill. Starting with the sump tank at the lowest point in the system, the system’s single pump pumps water up to the fish tank. From here, it flows by gravity throughout the rest of the system tanks. The next tank after the fish tank is the solids settling tank, then the fine solids capture tank (AKA net tank), then the degas tank, then the vegetable troughs, then back to the sump tank. All these tanks have constant water levels except for the sump tank. Even if you have some fish in this tank (which we do because, hey, it’s real estate we paid for!), varying water depths in this tank won’t cause the problems you would have if you tried to have varying water depths in the rearing tank, where the bulk of your fish live.

Why does the water depth vary in the sump tank? Well, if you put an auto-fill valve in this tank (the way we did at first) you will find that it’s always full. Then when it rains, the tank simply overflows out onto the ground and you lose the valuable nutrients in the aquaponics water that you paid for by feeding the fish. So we removed our auto-fill valves, stick the hose into the tank when the tank water level is about 6 inches above the water pump intake, and fill the tank about halfway. We made the mistake a few times of filling the sump tank to the top; this always makes it rain, just as washing and waxing your car does, and then all your nutrients go out onto the ground. If you install a small sump tank, you lose this flexibility, and will end up both having to fill it up frequently from the hose, AND losing valuable system water when it rains and the tanks overflows.

IMPORTANT! IF you are building your systems on a flat site, with no possibility of a lower location for the sump tank without putting an expensive concrete pit in the ground (to hold the sump tank, which is what some have done with their systems), you can simply pump from the end of the last hydroponics trough directly back up to the fish tank. You need to have a screen filter with 1/4″ to 1/2″ mesh here to keep crud and mosquito fish out of the pump, and your pump intake needs to be halfway down the end of the last trough, with NO standpipe as in the rest of your troughs. If you think about this, you’ll realize that this last trough is where the water level goes up or down with system evaporation or rainfall, because there is no sump tank for this to happen in. And you know you can’t pump water out of a standpipe if the water level goes down a bit below the top of the standpipe. This is why we put a pump intake fitting in the last trough that is always under water.

IMPORTANT! Fish tank drain is near TOP of tank!: We have a different design for our fish tanks than most, which have their drains in the bottom center of the tanks. Our tank drain is in the top 90% of the tank height, so that if there’s an earthquake that cracks the waterlines, we don’t lose all the water out of the tank and kill our fish. The “theoretical” reason for the center bottom drain in conventional fish tanks is to get all the solids out of the tank. Our “top-drain” design works because the air from the air stones in the tank churns up the water so aggressively that all the solids stay suspended in the water column and do not settle on the bottom of the rearing tank. We put goggles on and inspected our first fish tank after it was in service for four months to check on this and found NO solids anywhere on the bottom. Bottom drains add risk to these systems that is NOT necessary!

Fish Tank: The fish tank is the size it is for a specific reason: when you crowd tilapia they stop wasting energy on breeding. If the tank is bigger than about 4 gallons of water for each pound of fish in it, they will be more likely to exhibit breeding behavior, and will spend more of their energy to make eggs and babies, rather than just getting fat so you can sell them. This costs you more and doesn’t buy you anything. A bigger tank won’t hurt if you got a great deal on it, we’re just saying there’s no reason to buy a larger, more expensive tank than this “4 gallons per pound of fish”. Also, if the tank is TOO small the fish don’t thrive. We’ve found that anything smaller than 4′ by 8′ rectangular or 6-feet diameter for round tanks for full-sized fish seems to inhibit feeding activity and growth in our temperature range.

CRITICAL! Having a screen filter with window-screen-sized mesh between the net tank and the degas tank is critical for keeping tilapia babies out of your troughs! From our inspections of the net between the fine solid settling tank and the degas tank, it has never had eggs in it. We occasionally find a couple of small (2″) tilapia in the net tank, indicating they were hatched in a previous tank and didn’t get any farther than that.

Solids settling tank: Water enters this tank after leaving the fish tank, and the heavier solids from the fish poo settle to the bottom of the tank. If you make this tank too small the retention time (how long it takes for water going into this tank to exit the other end) is too short and you won’t get enough solids settling out, and these solids will flow through this tank (instead of settling out) and decay into ammonia somewhere else out in your system.

The water entering the solids settling tank becomes placid quickly. Install a drain fitting with a valve in the very bottom of this tank and drain 5 gallons of the resulting sludge off twice a day; put these gross (they stink!) organic solids into your compost or on your roses; they’ll love them!

A necessary part of the operation of the solids settling tank is to have 10-12 small tilapia (3-4″ long) living inside the tank. These fish scour the bottom sides of the tank and the resulting small turbulences carry the crud down into the bottom of the tank. If these fish weren’t there, the crud would accumulate thickly on the sloped bottom of the tank and not make it to the outflow. We tried one without the fish for three months and verified this. It doesn’t work without the fish.

Fine Solids Capture Tank: This tank removes fine suspended organic solids that are too light to settle out in the solids settling tank. These solids would decompose into ammonia in the system and might result in an excessive amount of toxic ammonia that would stress or kill the fish. The tank is filled with 3- 50-foot by 14-foot pieces of nursery netting, AKA orchard netting. The fine suspended solids entering this tank tend to be captured and held by the nursery netting, resulting in less crud entering the hydroponics troughs. When this tank is cleaned, from a couple times a week to once a month or so, the 100 gallons or so of crud that results goes down a pipe to the (optional) sludge decomposition tank where it makes more excellent organic liquid fertilizer. The fine solids capture tank used to accumulate a 2-3” thick solid mat of fish poop on top that you could set your coffee cup down on, it was so solid! Cleaning this tank used to be the worst stinking job on the whole farm, until the Gammarus showed up! They clean the net for us, now!

IMPORTANT! The first year we operated these systems the fine solids capture tank filled up with crud and required regular cleaning. After about 15 months of operation, we had some Gammarus (a water flea) show up in the system. The residence of these little beasties is considered by experts in to be an indicator of a high level of health in an aquatic ecosystem. After we found that out, and got over our initial worry that we had a new system pest to deal with, we were proud of our little Gammarus. They’re kind of cute.

Then we started seeing differences in the net tank; the first one was that there was a heck of a lot of Gammarus in the tank (they showed up in our oldest system first, we’ve successfully transferred them to all our other systems since then). At first we noticed the net tank wasn’t getting crudded-up as fast, then it wasn’t getting crudded-up at all! It’s been over six years since the first Gammarus colonized system #1, and we haven’t cleaned the net tank once since!

Even more telling than this was what happened when we transferred the Gammarus to system #2 in an effort to help them colonize it: when we started, this system’s net tank had a solid 3″ mat of crud floating on the top that you could set a full coffee cup down on. Over the next month and a half as we watched, the mat of crud gradually disappeared. We did some digging and found the net tank full of Gammarus, as if they had decided it was the Promised Land. As we seeded the other systems, they took up residence throughout each system: in all the tanks, in the hydroponics troughs, and in the roots of all the vegetables.

We don’t have a complete hypothesis explaining this yet. Our preliminary thoughts are that the Gammarus are affecting the crud that used to accumulate in the net tanks, and either eating it or breaking it down somehow so that it passes out into the systems. However, there hasn’t been any corresponding rise in nutrient or ammonia levels as one would expect if the Gammarus were simply breaking the crud up and it was going out into the troughs to decompose. But we haven’t had to empty our solids settling tanks or clean our net tanks in six years now. They’re just CLEAN.

IMPORTANT! There’s another equally interesting thing we’ve observed, that we will explain later in detail in the section about pH and water quality: we haven’t adjusted our pH once in any of our systems in the last two years. The last time we put calcium carbonate into our systems as a pH buffer was a year and a half ago, about four months after the Gammarus showed up. Although there’s complete information on Gammarus later in the manual, here’s a picture of our little friend so you don’t have to wonder any longer if you’ve already met one.

Degas Tank: The net tank used to be the nastiest place in the system before the Gammarus showed up. If you’re unable to establish Gammarus in your system it may be anaerobic in places or throughout, and generates hydrogen sulfide and methane (poisonous gases to the plants) in these anaerobic areas. So the next stop is the degas tank, where vigorous aeration blows off these gases as well as some of the carbon dioxide gas exhaled by the fish, before this water goes out to the plants in the troughs. There are four 0.5-cfm airstones in each degas tank.

IMPORTANT! The Degas tank MUST HAVE a screen filter to screen the water that comes in from the fine solids capture tank! This filter must be window-screen size or smaller, and is usually a wooden frame about 1 foot by 2 feet that is installed (removable so it can be cleaned) just below the inflow water from the fine solids tank. This filter catches the results of any breeding activity that may happen in the rearing tank so that tilapia babies do not get out into your troughs and EAT ALL THE PLANT ROOTS! If you have an LD system, there must be a filter like this on the fish tank outflow to the troughs!

Vegetable Troughs: Although the vegetable troughs are next in the actual sequence of flow in these systems, there’s so much stuff going on there that we’ll handle them in separate sections covering aeration and water flow requirements in the troughs (next section).

Sump Tank: Water flows by gravity back to the lowest tank (sump tank) after the troughs in a normal system. If your system is built on level rock, or on a tennis court, and it is difficult or impossible to install a sump tank lower than your troughs, you can omit the sump tank, put all the other tanks on the flat and put the pump between the last vegetable trough and the fish tank. If you have a normal system with the 900-gallon sump tank furthest downhill, you can use it as rental real estate: put 100-200 lbs. of fish into it. If you are going to have fish in this tank, you need to install an airline to it when you install the tank. When we have around 300 pounds of fish in this tank we need three or four 0.5 cubic foot per minute airstones in it to keep the DO up around 5 ppm. In fact, the fish in our sump tanks seem to be growing faster than anywhere else in our systems, another aquaponic mystery for someone to solve.

If you ever have to turn off the water pump you will need to follow a procedure so that you don’t lose system water by overflowing the sump tank. Why is turning the pump off necessary? Well, to do any system maintenance such as patch a hole in a trough liner, or repair a water pipe that you pickaxed (ask me how I know this 🙁 , you need to turn off the pump, and all that water has to go somewhere. If you just let it drain, all the excess water in the fish tank will drain into the troughs; then all the excess water in the troughs will drain into the sump tank. This can be thousands of gallons in a large system, and it will overflow the sump tank. Then, your system will be low by that much water when you restart the pump, and you will just have to fill it up with the hose.

So, here’s what you do: you run around putting temporary PVC caps and plugs on the fish tank outflow to the troughs, and on all the trough outflows (where each trough goes into the next trough in the system) right before you turn off the pump. This way, each trough’s water stays in each trough and you don’t lose any. You HAVE to remember to do this in reverse when you’re done with the repair. Take all the caps and plugs off, or you will simply pump all the water from the sump tank or last trough into the fish tank, where it will overflow.

Hose? You mean you’re putting chlorinated city water directly onto the fish? Yes: at first, we were super concerned about this, and would fill a makeup water tank with the hose, let it dechlorinate for a day, then pump or siphon it wherever needed. Because we often get rain in our location, we so rarely need to fill up it is easier to just put the hose in and run it while you do something else in the area. We haven’t seen any deleterious effects from chlorinated water on either the fish or vegetables. Also, our water is quite clean to begin with and only shows 0.25-0.5 ppm chlorine when it does have measurable chlorine in it. Check YOUR water; if it has high levels of chlorine you should know about it, and take measures as mentioned in this manual in the “Verify Source Water Quality And Fill Up” Section to dechlorinate it before adding it to the systems.