Introduction: Building an Aquaponic Grow Bed Flood and Drain.

Aquaponic grow beds come in all shapes and sizes. They also work on a range of concepts.


1. Flood and drain.
F&D beds are filled to a point that activates the Bell syphon. Once the bell syphon is activated, all of the water is sucked out of the grow bed. Once the water level reaches the bottom of the bell syphon, the syphon (water suction using gravity) breaks. This then allows the bed to start filling again.

Pros. Any kind of plant can be grown in this system.

Lots of balls for growing bacteria which helps to break down ammonia.

If you fill media beds with dirty water straight from fish tanks, the balls filter out the solid waste while the bacteria on the balls turns your ammonia into nitrite then your nitrite into nitrates. Nitrates are plant food and this cycle is what powers the entire aquaponic system.

Cons. You need a sump tank that holds enough water to flood all of your gorow beds at the same

2. Raft bed.
Raft beds are mostly used for growing leafy greens. The raft is normally made out of polystyrene +-5cm thick. Small holes are then cut into the polystyrene into which small pots are dropped. The plants then grow in these pots with their roots hanging into the water below. To allow for other plant types, bubble stones are sometimes used to provide extra oxygen to the roots of the plants in the raft. This allows a wider variety of plants to be grown.

Pros: Does not need a high capacity sump tank.
Easy to keep clean and very uniform layout.

Cons. Grow bed needs to be extra strong to hold the weight of the water 24/7.gym

Roots are susceptible to rot in the right conditions.

Step 1: Building Your Grow Bed.

First, measure the area where you intended the grow bed to live. Then decide if you are going to build it in place or in a separate work area.

You will need a table saw to cut the wood to shape. I cut my own wood but you can also get wood cut at places like B&Q.

Most plywood is sold in sheets 2 4m x1.2m I try not to have a join in the base of the box. Joins remove strength and that risks leaks. A 1.2m wide grow bed is also the widest possible bed to work if you can only access it from one side.

Choosing the depth of your grow bed is very important.. 40cm is best as it gives your plants space to grow and allows a layer at the bottom that breaks down fish waste very efficiently. Giving you space for an aerobic zone and an anaerobic zone. Over time this breaks down fish waste better and releases all of the nutrients in the solid fish waste.

Remember that your media beds have clay balls so a 400L capacity media bed only needs +-160L to flood and drain.

Step 2: Putting It All Together.

Putting these boxes together is very important. Make sure you use glue on every join and use more screws than you think necessary.

I recommend using a drill to make pilot holes. This limits the risk of screws puncturing the side wall of the box. In the second pic above, you can see I took a board and gave myself a line showing the thickness of the wood I was aiming to screw into, it makes it easier to drill pilot holes without missing the wood you are aiming for.

If you are building a box for a raft system then you need to use really strong wood. I personally made a mistake when I turned a media bed (F&D) bed into a raft bed. I didn't take into account the weight of the water.
I ended up cracking the seal and I had to add another side wall to hold the weight.

For a media bed (F&D) which is full of expanded clay balls, only 40% of the capacity is full of water, the rest is clay which is light so you can use 18mm ply base with 12mm sides. A 1000L grow bed would weigh approximately 550Kg. (including 400kg of water, 90kg of balls and +-60kg of grow bed wood.)

For a raft bed, it is full of water so that same 1000L grow bed weighs 1060KG. That needs thicker, stronger wood. So, I would recommend a 25mm base with 18mm sides.

Step 3: The Box Is Built, Now We Make the Drain Section.

OK, so, how do you male a water tight seal in a wooden grow bed?

It's harder than you think. so, after a few attampts and failures, I came up with this method.

For larger grow beds I recommend using 1.5" PVC waste pipe. Measure the size of the 1.5" PVC socket. Now drill a hole in the bottom of your grow bed, ideally in the corner that will sit above your sump tank. The hole must be approximately 3mm wider than the socket all the way around.

Now, find an offcut of wood. 15cm x 15cm is perfect. Now, drill a hole into the offcut that is slightly smaller than the socket. Now take a file and slowly widen the hole in the offcut. Eventually this hole will be the perfect size that the socket can squeeze into the hole.

Once the socket fits, use some 2 part epoxy to stick it into place in the hole. See the second picture. The socket should be nearly flush on one side and stick out much further on the other. The side that sticks out further is the top. If you don't have epoxy then use a screw, but epoxy is better as a screw can cause leaks.

Leave the offcut to one side for the epoxy to dry. Now you paint the box and then paint the offcut.

Step 4: Paint the Box Then the Offcut.

For my projects that hold fish or any other living organism, I use A1 Pond paint by Antel.
I spent so long looking for the right paint for these projects and I really do love this paint. Here's a link to the website.

It's a 2 part paint. Mix small batches at a time and NEVER put mixed paint back into the tin. The paint is not cheap but it works out to be cheaper than any other pond paint I've used because a little goes a long way.

Don't forget to paint the offcut. Try avoid getting paint inside the socket, you can however get away with getting paint on the outside of the socket.

When applying to wood, your first coat is to seal the pores in the wood. Don't go thick, just cover the entire area that will get wet. I always paint the inside of the box then the top of the side walls, which need sealing most and then a 10cm strip around the outside of the box. This is on marine ply which is really good with wet.. If you are using shutter ply then please consider painting the entire box, inside and out so your box lasts longer.

Remember to paint inside the hole in the bottom of the box. I also paint about 25cm around the hole on the underside of the box. (See picture 5.)

Your second coat should be done about 12-16hours after the first coat. I just leave it overnight. The second coat will start to look smooth and you can focus on sealing the corners inside the box.....

Wait another 16hours. You can move to the next step during your waiting for the paint to dry.

You will know if you want a third coat. I don't normally need one on marine ply but you might if you choose to use shutter ply.

Is the offcut looking smooth and glossy?
Yes good, now let's wait till everything is dry and let's put this all together.

Step 5: Building a Platform for the Grow Bed/s

So, if you are building this away from your sump tank then you will need to raise the grow beds off the floor. The extra height allows the water to run down to your sump tank.

For the base/stand I use blocks and bricks. First, measure where you need the box to sit. I chose to build two separate boxes to fit this space. I did this to avoid having a joint on the bottom of the box because the area was 3.4M long. Each box was 1.6m x1.2m...

Once you measured the area and decided where you need legs, you can lay out your blocks. If you are stacking blocks to make your legs, you need to make sure they won't move apart. I always fill one of the cores with concrete. Once dry, this becomes a solid leg and the blocks can't separate. I needed a little more height so I topped my blocks with bricks.

Now I have legs for my grow beds.

Step 6: Complete the Box.

OK. So, the A1 Pond Paint is now dry and the inside of your box is waterproof.

Your offcut is also waterproof and the pipe socket is stuck firmly into the offcut.

Drill a hole 3cm from each corner of the offcut and insert fairly short screws that will not break through the base of the box. Get one screw turned into place in each hole.

The top of the offcut is the side where the socket sticks out furthest. Now you need to push the top of the offcut onto the hole in the box. As before it should fit with a 3mm gap all around. It needs to sit perfectly central inside the hole.

Now that you are sure it fits perfectly, remove the offcut and apply a thick later of silicone to the top of the wood and around the socket that sticks through the hole. Once the fish safe silicone is in place, push the offcut into place, centre it then screw it to the box.

Silicone should squelch out around the sides and up through the hole. Wipe the silicone off the bottom of the box and inside the box, use your finger to smooth it around the socket. Don't dig it out the hole, rather let it fill the gap around the socket and wipe it off flush with the inside of the box. (See picture 5+6)

Once the drain pipe is in place, paint the outside of the box with a wood sealant. Doesn't need to be fish safe. Ignore this step if you painted the entire box with A1 Pond paint.

Step 7: Put the Grow Bed in Place.

Before you move the grow beds into place, wrap a strip of copper tape around the box. This stops slugs from climbing the side of the box and it helps aesthetically so the edges look good.

Now that they are in place, you need to decide if it will be a Media bed or a Raft bed.

Fortunately, I did one of each in this system.

Step 8: MEDIA BED.

So, you chose a media bed. To make this work, you will need to make a bell syphon. This is what drains the water out of the media bed.

A bell syphon is made of three parts.

1. The 1.5" drain pipe. The bed is 40cm deep so in order to stop it from overflowing, the drain pipe needs to be approximately 34cm long. This can be a straight unaltered pipe but I find that flaring the top end helps to activate the syphon quicker.

Flare the top by heating it with a blow torch then forcing s graded piece of wood into the hole at the top of the pipe.

Once done, the drain pipe can be placed into the socket. In order to make this fit snug, place a few layers of thread tape around the bottom of the pipe before putting it into the 1.5" socket.

2. You need a bell.

The drain pipe is 34cm tall so make your bell 36cm tall. For this you will need a 3" length of pipe that you cut 36cm long. You then use a saw or a file to create teeth on the bottom of the bell. Make the teeth approximately 2cm deep. This is where the water enters. When the water in the grow bed reaches 2cm deep, the syphon breaks, allowing the grow bed to fill up again. Make sure the teeth are all the same depth. If they are not uniform or deep enough, the syphon will not break and the grow bed will keep emptying.

3.The bell guard. 4" waste pipe.

Start with a 50cm length of 4" waste pipe.

Cut off a 5cm section of that waste pipe.
Now, cut strips up into the pipe, all ending at the same height (See Picture 6)

Once you have cut all around, take the 5cm section and put it inside the cut section. (See picture 7 and 8)

Place a cable tie around the outside of the bell guard, 2cm below the 5cm section. This locks the section in place.

Now, place the bell over the drain pipe then place the bell guard over the bell.

You now need to fill the grow bed with expanded clay grow media. Make sure you rinse the clay balls with normal tap water before adding them to the grow bed. The clay dust is not good for your system.

Place a brick or two on top of the bell guard. The clay balls move a lot for the first few weeks so you don't want the bell guard moving too.

Step 9: Raft Bed

For the rest bed you will need the stronger grow bed. The grow bed is always full to the desired level which you set by the length of your drain pipe.

Next, you need a sheet of 5cm thick polystyrene. First cut it to size to fit into the top of the tank. Next you paint it, I used the same A1 pond paint with no problems. Lastly you need a polystyrene cutter. Measure out your spacing for your holes, I chose 30cm between holes and each hole had a 5cm diameter that easily fit small net pots.


Once you have your pots in place, set a pipe to top up the grow bed from the corner opposite to the drain pipe. As water is added on one side it drains out the other side. The plant roots hang down into the water and with bubble stones on the bottom of the tank, you can grow pretty much any plant that can fit in the net pots.

Step 10: The Finishing Touches.

So, flow rates are a bigger issue in flood and drain systems. Ideally you should flood your grow bed 2-3times an hour. This doesn't take much flow so there is no need for super fast top up.

Finding the right flow speed for your bell syphon is very important. If your water flows too slow then you can have an issue where your syphon never starts.

If the water is too fast then your syphon may never break leaving your bed constantly empty.

If it's even faster then you may end up with overflowing beds. That's messy and ruins your plants.

Take the time to find the setting that works.

It's also easy, when planting in a media bed to forget how big plants really get. I made that mistake with my tomatoes this year. I guess we are always learning.

Fortunately, you don't need to grow seedlings separately. Just sprinkle your seeds on top of your media bed and most types will grow without an issue.

I grow my seedlings for my fast bed in a media bed. Mostly lettuce. I grow them for about 2 weeks then transplant them into the time net pots with a few clay balls for the roots to hold on to. They grow strong and fast.

OK, I think I've covered everything.

If you have any questions then please let me know and I will try to answer.

Aquaponics is an amazing hobby and we currently eat salad for one meal a day, most of the veg comes from my system.