Aquaponics Grow Bed: How Deep, How Big and What to Fill It With

An aquaponics grow bed is the container where your plants live, and in a media bed system it is also the filter that keeps the fish alive. Get its depth, size and media right and the rest of the system is forgiving. Get them wrong and you fight clogged gravel, yellow leaves and ammonia spikes.

This guide answers the three questions every builder asks: how deep the grow bed should be, how big it should be for your fish, and what to fill it with. The numbers come from the FAO aquaponics manual, Oklahoma State University, the University of Hawaii and Clemson extension, and every one of them is named where it is used.

What an Aquaponics Grow Bed Does

In hydroponics, the bed only holds the plants. In a media bed aquaponics system it does three jobs at once, and that is why FAO's manual (Somerville et al., Small-scale aquaponic food production, Technical Paper 589, 2014) calls media beds the most popular design for small-scale aquaponics and the one best suited to beginners.

• Mechanical filter. The media traps fish waste and uneaten feed before the water returns to the tank.
• Biological filter. Every pebble is surface area for the nitrifying bacteria that turn ammonia into nitrate. FAO says media beds have the most biological filtration of any aquaponic design.
• Mineralization. Trapped solids slowly break down into nutrients the plants can absorb, a process FAO notes is absent from raft and NFT systems.

The Southern Regional Aquaculture Center's aquaponics fact sheet (Rakocy, Masser and Losordo, SRAC Publication No. 454) frames the same point from the money side: combining the biofilter with the plant bed removes the cost of a separate biofilter, which it calls one of the main advantages of aquaponics. You will also see the grow bed called a media bed; they are the same thing.

How Deep Should an Aquaponics Grow Bed Be?

The short answer is 12 inches of media. The longer answer depends on what you want to grow.

FAO explains why depth matters: it sets the root space, and without 30 cm the larger vegetables run short of room, suffer root matting and nutrient deficiencies, and tend to topple over. It also adds that some experiments have grown large crops in shallow beds when the nutrient level was high enough, so 12 inches is the safe choice rather than a hard limit. Oklahoma State's Principles of Small-Scale Aquaponics (Mullins, Nerrie, Sink and Beem) puts most media beds at 6 to 12 inches.

The Three Zones Inside the Bed

A flood-and-drain bed is not one uniform block of gravel. FAO describes three layers, and they explain where the water level should sit:

• Dry zone, top 2 to 5 cm (1 to 2 in). Blocks light so algae cannot grow on the water, keeps the base of the stems dry to prevent collar rot, and cuts evaporation.
• Wet and dry zone, 10 to 20 cm (4 to 8 in). Where the water rises and falls. Most of the biological activity, root growth and bacteria live here.
• Wet zone, bottom 3 to 5 cm (1 to 2 in). Always wet. Fine solids collect here and are broken down by heterotrophic bacteria.

That is why Oklahoma State says water should rise to only 1 inch from the top of the media, and the University of Hawaii says 1 to 2 inches below the surface. Set the top of your standpipe accordingly.

How Big Should the Grow Bed Be?

Shape and Reach

FAO's standard media bed is a rectangle about 1 meter (a little over 3 feet) wide and 1 to 3 meters (3 to 10 feet) long. Beds should not be so wide that you cannot reach at least halfway across, and long beds tend to collect solids at the inlet end. Oklahoma State lists 4 by 8 foot lined wooden beds, IBC halves and barrels as the most used containers. Remember the weight: water plus gravel needs a strong frame, and FAO supports its beds on concrete blocks.

Sizing the Bed to Your Fish: the Feed Rate Ratio

FAO balances a system on how much fish feed goes in each day per square meter of grow bed, not on the number of fish. Its recommended rates, which assume a standard 32 percent protein feed:

• Leafy greens: 40 to 50 g of feed per square meter per day, planted at 20 to 25 plants per square meter.
• Fruiting vegetables: 50 to 80 g per square meter per day, at 4 to 8 plants per square meter.
• Fish eat about 1 to 2 percent of their body weight a day once they are over 50 g.

Here is what that means for common bed sizes. We converted FAO's metric rates; treat the ranges as a starting point, because FAO stresses that water temperature, fish size and the season all move the numbers.

FAO's own worked example lands in the same place: 25 heads of lettuce a week means 100 heads in the system, 4 square meters of bed, 200 g of feed a day and 10 to 20 kg of fish. When you choose the fish themselves, see our guides to the best fish for aquaponics and the fish tank.

When the Bed Is Too Small for the Fish

FAO warns that a bed undersized for the stocking density clogs with solid waste, which leads to poor circulation and anoxic spots. It recommends adding a separate solids filter if the stocking density goes above 15 kg per cubic meter of fish tank (about 1 lb per 8 gallons) or the feed goes above 50 g a day per square meter of bed. Its cheapest version is an old sock over the inlet pipe, rinsed daily.

Size the Sump for the Bed, Too

If your beds drain into a sump, FAO says the sump should hold at least 70 percent of the total media bed volume, because the media itself takes up 30 to 60 percent of the bed and the rest is water that has to go somewhere when the bed drains. Its example: a 1,000 liter bed (2 x 2 m, 25 cm deep) holds 400 to 700 liters of water, so the sump should be about 700 liters.

Grow Bed Containers: What to Use

FAO lists the requirements for any media bed: strong enough to hold water and media without breaking, able to stand the weather, made of food grade material that is safe for fish, plants and bacteria, easy to plumb, and close to the other parts of the system.

Frank Wood, who started AquaponicsDIYer, wrote that the simplest grow bed he knows is a shallow wooden box 6 to 10 inches deep, built like an ordinary raised bed and lined with pond liner. That depth suits greens and herbs; go to 12 inches if you want tomatoes. For a complete IBC build, see our IBC aquaponics guide.

Choosing the Grow Bed Media

FAO's criteria for a growing medium are a large surface area for bacteria, a neutral pH and inert material that leaches nothing, good drainage, enough room for air and water to move, and light weight if possible. It recommends particles of 8 to 20 mm: smaller gravel clogs with solid waste, larger gravel offers too little surface and plant support.

Oklahoma State adds two practical points: river rock is fine only if it has tested free of limestone, which would raise alkalinity and pH, and perlite is used less now because of its undesirable characteristics. FAO insists on washing any medium thoroughly first, because dust can clog the system and harm the fishes' gills.

Frank Wood's notes from his own build agree: he found perlite light enough to raise his beds off the ground easily, fine gravel cheap but heavy, and clay pebbles a good balance of the two. Our clay pebbles guide compares the brands and tells you how many bags a bed takes.

Flood and Drain or Constant Flow?

FAO describes three ways to water a media bed. Constant flow, where water trickles in and the level never changes, is the simplest, and FAO notes that some experts have shown it supports the same plant growth as more complicated methods. Flood and drain, also called ebb and flow, fills the bed and then drains it so the roots get both nutrients and air; FAO says these systems usually cycle once or twice an hour. And the timer method floods the bed by switching the pump on and off.

Flood and drain is usually done with a bell siphon, which needs no timer. Clemson's Aquaponics: System Layout and Components points out the bonus: because the roots are aerated every time the tray drains, the oxygen level of the incoming water matters less than in a raft bed.

Media Beds vs Raft and NFT

A grow bed is not the only way to grow plants on fish water. Oklahoma State and FAO describe two other common systems.

Oklahoma State says media beds are easy and inexpensive to set up and a good choice for small systems, while raft and NFT are more management intensive and costly for the beginning grower. FAO notes the flip side: media beds get unwieldy and relatively expensive at a large scale, which is why commercial farms lean toward rafts and NFT.

How to Set Up a Grow Bed

From empty container to first planting

1. Place and level it

   Set the bed on a frame or concrete blocks strong enough for wet media, above the fish tank or sump so it drains by gravity. Check it with a spirit level: FAO says keeping the bed horizontal is vital for the bell siphon to work.

2. Fit the drain

   Drill the floor for a bulkhead such as a Uniseal tank adapter and fit the standpipe at the height you want the water to reach, 1 to 2 inches below where the media surface will be.

3. Add the siphon and guard

   Stand the bell and the media guard over the standpipe before any media goes in. Sizes and the full build are in our bell siphon guide.

4. Test with water only

   FAO's advice: before adding media, fill the fish tank and sump and run the pump with the standpipe removed, so water flows straight through, and fix any leak you find.

5. Rinse and add the media

   Wash the media until the water runs clear, place it around the guard first so it stays put, then fill the bed to 1 to 2 inches above the top of the bell.

6. Plant seedlings, not seeds

   Oklahoma State notes that direct seeding does not work well in aquaponic systems; transplant started seedlings. Start with greens and herbs from our best plants for aquaponics list while the system is young.

Keeping a Grow Bed Healthy

Add Worms

FAO and the SRAC fact sheet both describe adding red worms (Eisenia fetida, the composting worm) to media beds. They live in the wet and dry zone, eat solid waste and dead roots, and help stop the bed from clogging.

Watch for Clogging

SRAC warns that gravel and sand beds overloaded with organic matter can start producing ammonia instead of removing it. If water starts to pool on the surface or smells sour, Oklahoma State's guidance is that media beds need periodic cleaning or they clog and create anaerobic zones; reduce feeding, add solids filtration, or wash the media.

Clear Out Roots After Harvest

Oklahoma State reminds growers to remove all plant material, including roots stuck in the media, when they harvest. Old roots are one of the things SRAC lists as a cause of clogging.

Test the Water Weekly

The bed is your filter, so the water tells you how it is doing. Oklahoma State recommends weekly tests of pH, dissolved oxygen, temperature and nitrogen, and a pH of 6.8 to 7.2 as a common compromise for fish, plants and bacteria. A liquid kit such as the API Freshwater Master Test Kit covers pH, ammonia, nitrite and nitrate. If the system is new, follow our fishless cycling guide before adding fish.

What you need for one grow bed

A complete flood-and-drain bed of about 11 square feet with roughly 10 inches of media. Swap the container for a cut IBC or a lined frame if you prefer.

1. 

   1Grow bed container

   Rubbermaid Commercial Brute Stock Tank, 50 Gallon

   The listing gives 52 x 31 x 12 inches: the 12 inch height FAO recommends for fruiting crops, in a rigid, ready-made tank with no cutting.

   Check price on Amazon

2. 

   2Grow media

   Mother Earth Hydroton Original Expanded Clay Pebbles, 50 Liter

   Expanded clay that the maker lists at 8 to 16 mm, inside FAO's recommended 8 to 20 mm. A 12 inch deep bed of this size takes several bags.

   Check price on Amazon

3. 

   3Bell siphon

   Smoky Mountain Aquaponics Bell Siphon Kit for 8 Inch Media

   The listing describes a 2 inch bell 8 inches high, a slotted guard, threaded bulkhead adapters and elbows; the right height for a 12 inch tank, where a 12 inch kit would stand above the rim.

   Check price on Amazon

4. 

   4Inflow valve

   Homewerks 1 Inch PVC Schedule 40 Ball Valve

   Goes on the line from the pump so you can trim the flow until the bed fills and drains on a steady cycle.

   Check price on Amazon

5. 

   5Water testing

   API Freshwater Master Test Kit

   Liquid tests for pH, high range pH, ammonia, nitrite and nitrate, the numbers that show whether the bed is filtering.

   Check price on Amazon

Aquaponics Grow Bed FAQ

Keep building: bell siphon, aquaponics pump sizing, IBC aquaponics and the full how to build an aquaponics system guide.