Low Density Systems

“Low Density” refers to the relatively low density of fish required for excellent vegetable growth. Our Low Density (or LD) systems only require four major components: a fish tank, vegetable troughs, a water pump, and an air pump or blower. Water starts out in the fish tank and flows by gravity to the first hydroponic trough; down the length of that trough then across to the second trough, etc., and on through however many troughs in the system to the last trough; then it is pumped out of the far end of the last trough and back up to the fish tank. Each and every single vegetable trough has a standpipe at the OUTFLOW end of the trough that regulates the water height in that trough (EXCEPT for the outflow from the last trough, which we’ll address in a moment). This works because water is very simple stuff: it gets up to the top of the standpipe and simply flows down the open hole it finds there.

(Below) A nice 256 square foot LD system outdoors in subtropical Hawaii.

IMPORTANT! The water intake in the last trough is in the middle of the end, halfway from the bottom of the trough to the top, because water height will vary in the last trough; it gets higher when you add water to the system and when it rains; and it gets lower as evaporation occurs and water is turned into vegetables (weight and volume) by the system. You do NOT want to let the water get below this intake, because then the water pump will suck air and stop pumping water. If you let these pumps suck air, it gets trapped in them and then they stop pumping water, EVEN IF YOU REFILL THE TROUGH! The trapped bubbles in the pump just keep going round and round, and the only fix is to refill the trough, turn the pump off, and let the refilled trough water flow through the pump to reestablish the pump’s prime, then turn the pump back on. If a trough gets dangerously low (within 1” of the intake fitting or so), you need to add water to the system before you suck air into the pump and have to reprime it.

Here are the components and sizing for a 256 square foot LD system:

256 Square Foot LD Family Aquaponics System Proportions

These systems are simple to build and operate, and grow just as many vegetables as our higher-cost HD systems. They are bulletproof: we started operating our first LD system at the beginning of the worst winter in the last 33 years. At system startup we had 20-ppm nitrates, which went down to zero three weeks later. It was cold and gray for five months, and during that time we got 60 inches of rainfall, which diluted the system’s nutrients by dumping the system water out on the ground many times. Yet even with these problems with unmeasurable levels of nutrients, we had the same vegetable production from this LD system as we got from our more expensive HD systems. We suspect these systems will run on much less than the 0.3 pounds of fish per square foot of raft we are currently recommending as a minimum stocking level, but we consider that level safe if your troughs are outside a greenhouse and subject to dilution through rainfall as ours are.

Our LD systems work at such a low level of electrical consumption because they’re stocked at a fish capacity of 3/10 of a pound per square foot and only need their water pumped during daylight hours. We ran an experiment in which the water pump in our Family-sized system (80 lbs. of fish in 1,600 gallons of water) was switched off using a $15 Ace hardware-store timer during the dark 12 hours of every 24. Although we chewed our fingernails the first couple of nights, six months later this is working just fine. There is no apparent drop in vegetable production from a nearby system in which the pump runs 24/7, and which is growing the same items. This setup drops the total power consumption of the system to 0.7 kW per day, which is 60 watts for 12 hours out of every 24. Our 256 square foot Family LD system uses an Aquatic EcoSystems MD2 water pump that consumes 24 watts, and an Aquatic EcoSystems TL40 air pump that consumes 35 watts.

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. Well, actually, there is an exception to this; that’s if you have a trust fund. Then you can raise all the fish you want without needing to be concerned with profitability.

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 kilowatt-hours 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 kilowatt-hour, 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!

Optional Sump Tank: this is a large tank situated downhill from the last trough to catch the overflow of system nutrient water that comes from the troughs when it rains. If you install a sump tank, you will pump water from the sump tank back up to the fish tank instead of from the last trough.The water flows by gravity from the fish tank to the troughs, then back to the sump tank.

Optional Overflow Tank: This is a large tank situated downhill from the sump tank to catch the overflow of system nutrient water that comes from the sump tank when it rains. Think about it: when it rains, all the water goes into the troughs. They drain by gravity flow into the sump tank (if you have one). So when it rains, you’re going to lose nutrient water as soon as the sump starts overflowing.

If you have the sump overflow plumbed to yet another tank downhill (what we call the “overflow tank”), all your good nutrient water will go there, and can then be pumped back into the system instead of using potable or catchment water to make up system losses. Or you can just use the nutrient water in the overflow tank to irrigate things on the farm. They’ll grow like crazy.

The only sneaky thing is that both these tanks only make economic sense when your farm has a slope so that there is a “downhill”. Digging a hole to put a tank in is expensive, and the only kind of tank that doesn’t collapse or rot away inside a hole is a concrete one, which is also expensive!