Charcoal for aquaponics?

Hi guys,

I recently found a bunch of carp fingerlings in our pond (well more like a puddle in the swamp). They have no chance of surviving the winter so l decided to catch and transfer them. I have 13 of them in a 40l fishtank in the house, but there are still about 30 in the pond.

I was interasted in aquaponics for a while, and now l happen to have the opertunity to try it out. In my greenhouse there is a ~1m2 space thats always in the shade, nothing grows there. I will put the ~500l fishtank there, maybee eaven 2 250l, and make one part of the bed aquaponic.

My question is this. As l understand, a growing medium is required that has as much surface area as possible. People use gravel or clay puffed balls, but l was thinking, wuldnt charcoal be ideal?

I guess there might be a problem with pH. Any guesses?


Hi Kristijan,

The charcoal might be to much base for the water ( increasing PH ) but you could wash/boil that out.
What is the original material or how is the charcoal made ?


Yes thats my fear allso. I guess its the same as with biochar, it takes a while for the microorganisms to breakdown/neutralise the basic compounds.

Hmm boiling might do the trick! Perhaps add a bit of vinegar to the water and then wash out?

Well l wuld make the charcoal in the Kursk kiln. Its high quality ringing char.


Hi Kristijan, have you seen these systems?

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You should wash the charcoal out, and IF your goal when making it for your hydroponics, I would consider putting out the fire with water, which will expand the pores a bit. Even if you don’t, you will still want to sift off the ash, then wash it with water.

I snagged this comment from a blog post about why not to use biochar as your growing medium.
“I did this very experiment and saw a reduction in growth with different types of biochar. However, if you pre-incubate the biochar for 3 weeks in fish effleunt/waste bringing the biochar to equilibrium, you will find that it will not inhibit growth. The same holds true for soil addition, it is an addition with long term benefits. I can imagine a situation that it would be beneficial in Aquaponics . For example, if your trying to save energy by having few flood and drain cycles, having a media with high CEC properties that will trap nutrients near the root zone may be beneficial. But there has been no definitive experiments done to date that I know about proving its effectiveness in media-based systems.”

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I think there’s minimal factual information regarding biochar function increasing fertility.

We do know a few things though. Fresh char contains strong bases, so it will naturally cause PH and nutrient flow upset in sensitive systems. This probably explains the issue with the potting soil example above. In most agricultural land the natural buffering will easily negate the change in PH.

There seems to be a lot of mystique around composted biochar, and unsupported claims aimed at marketing and profits. Of course marketers want to substitute as much cheaper or inert material as possible in their products. Mixing with compost probably also helps eliminate problems at the retail level with people using it in plant pots, washing and buffering issues, apart from the fact char is pretty inert, and not really beneficial in pure form, like perlite or vermiculite.

In many soils char will help retain moisture, and certainly is porous, forming habitat for soil organisms, this could indirectly help with nutrient availability. The presence of carbon may influence nitrogen cycling in the soil.

For sure it’s a carbon sequestration strategy, probably of modest fertility enhancement, the largest effect may be water retention. (Which will also assist nutrient availability in arid conditions)

Some studies have found fertility reductions in soil around 155 T/Ha.

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If you wash it, you get rid of the KOH which lowers the pH. I agree there is a mystique around biochar, but charcoal itself was used as a potting soil filler up until like the 70s when the treehuggers got involved, instead of perlite or vermiculite.

There is a lot of mystique surrounding it. If you drench it in water to stop the burn, they throw a lid on, it becomes softer and closer to activated charcoal, which is more porous. It HOLDS water for quite a long time. It has an extremely high CEC (cation exchange capacity), so it does hold nutrients.


I’ve been advocating charcoal as a substrate in aquaponics for years but have had little success getting anyone to try it. As others have noted, make sure you wash the charcoal and use some acid (vinegar) to neutralize the carbonates and hydroxides that are the ash component of the char. The charcoal I make has a pH of 8.5 to 9. This is due to the ash and is rather quickly neutralized when applied to the acidic soil in my neck of the woods. However, when putting it in direct contact with fish, you want to make sure the pH is near 7 or slightly less. Well cooked charcoal is mostly carbon that will hold nutrients and water. If it is finished at a high heat (red hot) the tars, aromatics and oils will be totally cooked off which is what you want in an aquaponic situation.
Gary in PA



For some reason the link doesent work?

Thanks for the input guys. Good to hear from from you too Gary.

I think l will give it a try. Perhaps do 2 beds, one with clay balls and one with charcoal, for comparison.
My guess is charcoal might do much better as a biofilter, and as a grow medium for peranual plants. In my experiances with biochar l see plant roots love to grab on charcoal and this might be a problem when a plant needs to be pulled out of the bed.


There was a thread on this in “Backyard Aquaponics” I know when I visited a large aquaprium shop they had a 3 foot tower of charcoal they would cycle all the tanks in the shop to purify the water with.

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Activated charcoal is used a lot as a filtration for coy ponds as a 3rd filter after the sand filter. It picks up the ammonia and apparently pheromones that this fish give off that stunts their growth. I just found that out while looking up ways to clean up a pond. :slight_smile:


It’s getting a bit off topic, but I read barley straw naturally inhibits algae. Not sure if it’s toxic to fish, or how inhibitory it is to algae.

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Don’t know why, works on this end. search hydroponic gardening, uses fish waste to feed plants, plants to clean water for the fish.


I heard that too. It inhibits algae, but it is non-toxic to fish. Finding barley straw is a whole other issue at least around here. You probably have an easier time with it.

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I think the word you are looking for is aquaponics.

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Did you ever give this a try? If so, what were the results?

I think once neutralized it would certainly be a cheaper option for substrate. Definitely cheaper than puffed clay balls. I really want to get back into aquaponics, I have a pond full of fish native to the area so they are perfect for outdoor growing. Bluegill and Bass of a few varieties. If I can find a native fish that tastes like flounder I’ll be really happy.


Sean, l did not. I tryed one small sistem with young carp and gravel as the growing medium but decided aquaponics are not for me :smile: if the pump dyes or pluggs the fish dye if not found soon enaough, plants dry out, fish need to be fed just right to feed both fish and plants well, water temperature needs to be controlled… With clasical gardning you throw seeds in the ground and it preety much works on its own :smile:

But aquaponics did teach me a valuable lesson! Never fight aginst nature and try to force our human perfection to things. Clean weed and pest free garden beds are not natural. Let me explain.

In the back of the greenhouse l had the dug in hidroponic fishpond. A about 20gal plastic tub. Reminence of the aquaponics sistem l never bothered to throw out. Green smelly watter inside. I always had slug problems in the greenhouse, litle white ones that hide so well in daytime and devour crops in the night. Impossible to get rid of. But suddenly, they were all gone. What happened???
A evening visit to the greenhouse revealed 2 golden eyes looking at me from the dirthy tub of water. A toad found its home in the greenhouse! And guess what toads eat…
As a resault, l allso learned about deep mulch because l didnt want the toad to feel dry and provided her with a thick matt of hay over the beds. Then the weeds went away because of the mulch!


For sure, it isn’t for me. :stuck_out_tongue: I never got the numbers to work favorably. Hydroponics always had the same issue to me, unless you were doing a high margin crop like marijuana. The kratky method skips over a lot of the expenses so I am going to try that for some veggies this year.