Aquaponics - Grow beds

On day 4 I made life! Pictured is a coz lettuce 4 days after the seeds were sprinkled around.

When you first start running your system you do it without fish because the environment is too unstable. You can do it with fish but have to be very careful to avoid a stinking mess and some major karmic debt.


I'm happy to eat fish (and can cope with any fish deaths that may occur as a result of my eating them) but don't trust my new abilities as a fish farmer, so I'll be doing what is called fish-less cycling to get my system started. That doesn't mean you can't put plants in because, it turns out,  to cycle fish-lessly you add ammonia in some other form, and the entire process works just fine without the fish.. I'll talk a bit more about fish-less cycling in another post.


 A garden bed or grow bed (abbreviated in online forums as GB) is filled with some kind of gravel-like media so the plants have something to hold onto, and so the nitrifying bacteria have somewhere nice to set up house.

It seems plants don't actually like the exhaust that fish put out, but bacteria exist that love to convert it into stuff other bacteria like to convert into the kind of thing plants like.

Fish excrete ammonia, (fish don't like ammonia) some bacteria change that into nitrites (fish don't like nitrites either) and then some other bacteria convert nitrites into nitrates. Plants seem to like nitrates and fish don't hate nitrates quite as much as they hate all that other stuff. This process is known as the nitrogen cycle, and more information can be found here by clicking this.

The gravel that you use depends on how wealthy you are. You can buy clay balls that look nice, are great to work with, and work very well. Or, as I have you can use scoria (some kind of volcanic rock with lots of holes in it). you can also use gravel.

I used scoria (and a small amount of an experimental clay based media I made) because it cuts your hands and is difficult to work with. I'm not very wealthy. It was cheaper and it has a stack of trace elements that should make themselves available to the plants over time. My scoria is also red, most red stuff in rocks in Australia (I'm in Australia) is iron. Plants like iron. Scoria also has a huge surface area because of all the holes and because of its irregular shape, so there should be plenty of space for the bacteria to colonize and lots of cavities to hold water.

scoria looks like this (scoria doesn't always have a key in it)...

- the beasties live in the grow media. They eat fish crap and crap out plant food.
- interestingly a stack of fish eat plants, and those that don't, tend to eat things that do, so the entire thing can just go around and around for ever. Which is nice.

Those bacteria also eat fish food (or at least they eat the stuff that fish food will break down into), so any food not eaten eventually breaks down and gets absorbed nicely into the system (within reason).

By adjusting the height of the siphon's standpipe its possible (and desirable) to set the flood depth of the grow bed. Set high tide to a point just below ground level. I'm told 2.5cm below ground level is about right. Plants don't like to get too soggy and the bacteria don't like light so there is no point in over filling and its just going to waste more water to evaporation if you over fill the grow beds (I'm told aquaponics uses only about 10% of the water you might use on a dirt garden). Its also a good idea not to fill your grow beds all the way to the top with gravel either. Over filling your beds with gravel will end up with you spilling your media onto the ground every time you dig around or harvest a plant.

My grow bed takes around 20 minutes to fill, and around the same time to drain.  The pump runs all the time so water is flowing in the entire time, even when its also flowing out.

As a rough guide your media takes up around 60% of your grow bed leaving space for water in the other 40%.