Articles and Information

"Build a Mini Aquaponic System"

by Rebecca Nelson

A mini aquaponics system is an excellent means of demonstrating aquaponic principles and the nitrification cycle in a recirculating aquatic environment.  Following are instructions for building a small system that is ideal for a teacher, students of hobbyists who wants to get a start in aquaponics.  

What You’ll Need:

Following is a list of the parts you’ll need to build an aquaponic system. The next section, Components Explained, describes and explains each of these components and includes recommendations for alternative items and specific products.

• A tank for the fish: 3-20 gallon, glass or plastic container ($5 – $20)
• Gravel – 2.5 lbs./gravel for every 5 gallons of water in the fish tank ($2 – $5)
• Water pump – 3-4 watt pump capable of lifting 18” – 54” at 30 – 100/gal/hour (small circulation or fountain pump is ideal) ($19 – $40)
• 3 ft. of plastic tubing that fits the outlet on your water pump ($1 – $2)
• Aquarium air pump sized for the number of gallons in your fish tank ($8 – $16)
• Air stone (1” – 3”) ($1 – $2)
• 3 ft. of air tubing to connect the air pump to the air stone (must fit the air pump outlet) ($ 1)
• Grow Bed – must sit on top of fish tank and be 3” - 8” deep ($ 5 – $20)
• Growing Medium – enough pea gravel, perlite, coconut coir, expanded clay pebbles or peat moss to fill the grow bed ($2 – $5)
• pH test kit and, depending on the pH of your water, pH down or pH up ($5 – $15)
• Fish and plants

Tools Required:

• Drill with 1/4” or 3/16” bit and 1/2” bit

• Scissors
• Electrical tape

Component Explanation:

a tank for the fish
The fish tank can be a glass or plexi-glass aquarium or you can use any other clean container that holds water, for example, a plastic tub, bucket or barrel. We recommend anything between 3 – 20 gallons, although, you can go with a larger tank if you have the space. Small, clean plastic amphibian cages, available in most pet shops, make an excellent mini-system. They hold about 3 gallons and are quite inexpensive.

The standard sized fish aquariums of 10 and 20 gallons are also reasonably priced. The larger the tank, the larger grow bed area you can support. As a general rule, you can support 1 - 2 square feet of growing area for every 10 gallons of fish tank water.

gravel for tank bottom
The gravel serves as a home to the nitrifying bacteria that convert ammonia to nitrite and then to nitrate, which can be used by the plants. Most pet stores carry natural or colored aquarium gravel. The individual pebbles are about 1/8” in size. Be sure to wash the gravel thoroughly before using it because it is often dusty. Unwashed gravel will cloud your tank water.

water pump and tubing
A small water pump is used to pump the water from the fish tank to the grow bed. After the water is pumped into the grow bed, it gravity-feeds back to the fish tank. You’ll need enough tubing to go from the outlet on the pump to the top of your grow bed and form a circle within it.

air pump, air stone and tubing
You need an air pump to blow air into the tank water for both the fish and the plants. Tubing connects the air pump to an air stone at the bottom of the tank.  The air stone breaks the stream of bubbles coming from the air pump into micro-bubbles, which greatly increase the oxygenation in the water.

grow bed
The grow bed, which sits on top of the tank, must be slightly larger than the length and width of the fish tank. The grow bed is filled with a growing medium that the plants grow in. A plastic Rubbermaid container, a garden planter or other container that will sit on top of the tank will work fine. The container should be between 3” - 8” deep.

You can use a plastic tub or, for a very nice looking unit, build one out of plexi-glass and seal it with a non-toxic, silicone glue. If you build the grow bed, you can accommodate an aquarium light by making a cavity in the grow bed that the light can slide into. If you are using some other kind of container, a light can sit just behind it if there is room.

growing medium
A growing medium is a porous, chemically inert material that holds the plant roots and maintains moisture. Examples include: perlite, expanded clay pebbles, peat moss, pea gravel and coconut coir. You need enough to fill your grow bed.

fish and plants
In an aquaponic system, the fish provide the nutrients the plants need and the plants purify the water by consuming those nutrients.

aquarium heater (for tropical fish)
Most gardeners or aquarists setting up an aquaponic system choose ornamental fish for the tank and most ornamental fish originate in tropical waters. A tank temperature of 78 degrees F will need to be maintained for tropical fish. Two kinds of aquarium heaters are available, submerged and tank-side mounted. Either will work, but be sure the heater you choose is sized for the number of gallons of water in your fish tank. If the aquaponic system is placed in an area where the air temperature is maintained between 70 – 76 F or, if you choose cool water fish goldfish, you do not need a heater.

light for fish tank
Most aquariums have a florescent light so you can see the fish and monitor their health. You can add one if you’d like but it is not a necessity.

grow light for the plants
If you establish your system in an area with low light levels, you may need to add artificial light for healthy plant growth. Keep in mind that bright light will quickly encourage algae growth in the fish tank. You should try to point an artificial light in a way so that it does not directly penetrate the fish tank. If you do have rapid algae growth, you can scrape the interior walls of the fish tank or buy a plecostomus, a fish that eats algae. If the grow bed is in a windowsill with bright sunlight, in a greenhouse or planted with plants requiring low light levels, a grow light isn’t necessary.

Step 1.
Thoroughly wash the gravel and place in the bottom of the fish tank.

Step 2.
Drill 1/8” or 3/16” holes in the bottom of the grow bed every 2 square inches so the water can drain into the tank. In one of the back corners of the grow bed, drill a 1/2” hole for the tubing from the water pump to pass through.

Step 3.
Place the water pump in the fish tank then set the grow bed on top of the tank. Feed the tubing from the water pump through the 1/2” hole. Leave enough tubing to extend about 3/4 the height of the grow bed and to loop around the inside of the grow bed. Cut off any excess tube and fold the end over. Seal the folded piece with electrical tape.

Step 4.
Fill the grow bed with the growing medium to just under the top of the tube.

Step 5.
Puncture small holes every 2 inches in the section of tubing that loops in the grow bed.

Step 6.
Cover the loop of tubing with an inch or two of growing medium.

Step 7.
Fill the fish tank with water.
Plug in the pump to ensure that the water is pumped into the grow bed, trickles down through the growing medium and continuously back into the tank. Depending on the size of your tank, grow bed and pump, you may have to adjust to flow.

Step 8.
Connect you air pump to the air stone with the air tubing. Place the air stone in the tank and plug in the air pump. A steady stream of bubbles should rise through the water, providing fresh air

Step 9.
Check the pH of your water using litmus paper, a pH test kit or pH meter. Limtmus paper and inexpensive pH test kits are avilable in most hardware pool supply stores. The ideal pH is 7.0 for an aquaponic system. If it is higher than 7.2 you should lower it with a “pH down” product and if it is lower than 6.8 you should raise it with a “pH up” product, both of which are available from aquarium stores.

Step 10.
Allow the unit sit for 24 hours to be sure all chlorine has dissipated from the water. If you want to stock you fish right away, you’ll need to add a chlorine remover, which is available from aquarium shops and pet stores.

Step 11.
Add your fish to the fish tank. Initially, you should lightly stock your tank with no more than 1/2” of fish per gallon of water. Once your system has been established for over a month you can increase to fish density to 1” per gallon of water.

Step 12.
Ideally you should wait approximately 4 weeks to add plants to your system, but if you are eager to plant it, add just a few plants or seeds and increase plant density in a month or so when your system is well established.

Fish and Plant Selection

In selecting your fish, choose hardy species like goldfish, guppies, angelfish and other common varieties available from your local aquarium or pet store. Most desktop aquaponic gardens do not include food fish because there isn’t enough space to grow them to maturity. If you do want to raise food fish or a local species, be sure to provide adequate water temperature and feed.

A desktop aquaponic garden will support most varieties of house plants, lettuce, spinach and herbs. Ideally, you should start your plants from seed in a grow cube (also called jiffy cubes) or loose in the growing medium in your grow bed. Very small seed can be sprouted by placing them between two paper towels that are kept warm and most. You can also transplant plants from an existing hydroponic system with good results.

If you must transplant from soil, thoroughly wash away all of the dirt surrounding the roots and wash the leaves being sure to remove any pest insects.

You will have the most success with leafy vegetables like lettuce, spinach and herbs or houseplants such as anthodium, dracaena, dieffenbachia and philodendron.

You can also plant aquatic plants in the fish tank. They will provide a more natural habitat for the fish and aid in purifying the water.

Fish excrete ammonia in their wastes and through their gills. In sufficient quantities ammonia is toxic to plants and fish. Nitrifying bacteria, which naturally live in the soil, water and air, convert ammonia first to nitrite and then to nitrate. In your aquaponic system the nitrifying bacteria will thrive in the gravel in the fish tanks and in the growing medium in the grow bed. Nitrate is used by plants to grow and flourish. The plants readily uptake the nitrate in the water and in consuming it, keep the levels safe for the fish.

The only daily input in this system is fish food. With any aquarium, frequent small feedings are better than fewer large feedings. Unless you have a really large tank, a pinch of food is all it takes. You should never feed more than the fish can completely consume in 5 minutes. Most tropical fish will be fine with a dry flake fish food but occasionally varying their diet with brine shrimp or blood worms will definitely keep them healthier and happier.

The water level in the tank will slowly decrease as some water is absorbed by the plants and some evaporates. Every few days you should refill the tank to the top. About once a month a 10 - 15% of the tank water should be siphoned out and replaced with fresh water.

An aquaponic system is an excellent tool for experimentation and proving or disproving a hypothesis. Following are four theories and experiments that can be done to prove each.

Theory 1:
Although an aquaponics system will produce good plant growth, the hydroponic system with precisely measured nutrients will produce faster growing, higher quality plants.

Experiment 1:
Set up a hydroponic system and an aquaponic system. Monitor and document which one best supports plant growth.

Theory 2:
A healthy aquaponic system has ample nutrients for leafy crop growth, but fruiting plants will be lacking sufficient quantities of certain elements.

Experiment 2:
Plant a leafy crop such as lettuce and a fruiting crop such as tomatoes and monitor to see which one does best in aquaponics.

Theory 3:
A pH of 7.0 is the best for an aquaponic system. At a lower pH, nitrification slows down and the water quality will be reduced, stressing the fish, and at a higher pH the plants will be stressed.

Experiment 3:
Set up three aquaponic systems. Run each at a different pH, one at 6.0, one at 7.0 and one at 8.0. Observe and document the plant growth and fish health at varying pH levels.

Theory 4:
Denser fish populations will support more plant growth due to increased fish waste and nutrients in the water.

Experiment 4:
Set up two aquaponic systems, stock one with 1” of fish/gallon of water and the other with 1/2” of fish per gallon of water. Observe the difference in plant growth.