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My grow bed needs cleaning out!... Maybe Not.

Discussion in 'General Aquaponics discussion - Aquaponics system ' started by Yabbies4me, Dec 6, 2017.

  1. Yabbies4me

    Yabbies4me Administrator Staff Member

    Jul 27, 2010
    I've seen many posts across a number of forums, Facebook chat groups etc, about people cleaning out their grow beds after only a year or two of operation. On the other hand I've seen many posts and had feedback from customers saying that they've had beds running for 4, 5, even 6 years without needing to be cleaned out.

    What I'm beginning to realise now is that there is so much discussion around about "muck" in grow beds being a bad thing that many people are assuming any build up is bad, but that's simply not true if you have worms in the system.

    I regularly show customers the build up of worm castings in the grow bed in my display system and I've had a few customers tell me they'd cleaned out beds with much less build up because someone online had told them it needed it. When I tell them that my grow bed has looked like that for at least 3 years they are shocked.

    A bit of background on my system: I built the system nearly 5 years ago and it was built the way it was (a basic system with no sump or mechanical filtration) because there was a lot of discussion around at the time about whether to filter fish waste solids from the water prior to the grow bed or not. Much of the pro-filtering conversation was initiated by one particular contributor to a number of forums who described that particular basic design as the "Sewerage system" and stated that such a system wouldn't get 12 months without the GB needing cleaning out and/or the fish suffering severe health issues or dying... Even though many people had been running basic systems very successfully for many years prior to the mechanical filtration revolution... So I decided to build one exactly how he said not to and see how many years it would take before the GB needed cleaning out.

    The system has been running nearly 5 years now and it is yet to develop anaerobic zones in the GB. I had Trout in there initially and it currently contains Silver Perch that are 4 years old, which I have progressively harvested since they were about 18mths old. I just harvested one recently and it was 1.75kg

    If you look at the pics below you'll see there is definitely a build up of "muck' in the GB, but it is worm castings and they are mostly in a layer up around the max flooded level, so about 40-50mm below the surface of the expanded clay gravel. The worms are feeding on old roots left behind after harvesting plants, as well as venturing down to the base of the GB and feeding on the fish waste before bringing it back near the surface and depositing it as nutrient rich castings.

    I tend to find pockets of the castings, obviously there are particular spots in the bed that the worms like more. When I'm harvesting plants and find one of these pockets that is thick with castings I like to give the expanded clay in that area a clean by:

    - Turning off the pump when the GB has just finished a drain cycle.
    - Digging out the mucky clay and dropping it into 1/3 of a bucket of system water, until the bucket is full.
    - Swish the clay beads around to clean off the worm castings.
    - Scoop out the clean beads into another empty bucket.
    - Add the clean beads back into the GB.
    - Turn pump back on.

    I'll usually only do 2-3 bucket loads of gravel in one area and at one time. I don't like to do too much in one hit as it can make the water in the system murky.

    The left over bucket of murky water full of worm castings (aka: nutrients) I take home, water down 50/50, then add it to my garden, around the rose bushes, palms etc.

    So when do grow beds need cleaning out?... Basically if you have a layer of muck built up in the base of your GB and if disturbed it smells like a camel’s bum, and the pH of your system has stopped dropping or started to climb after years of declining, then your GB has developed an anaerobic layer and should be cleaned out completely. When cleaning it out you should ensure that none of the disturbed sludge gets into the water in the system, the smell coming off it is Hydrogen Sulphide, which kills fish very quickly.

    Grow beds could also need cleaning out if they are simply clogged with old plants roots that are restricting water flow through the bed.

    When do grow beds NOT need cleaning out?... If the he pH of your system is still naturally declining (normal for an established system) and there are worm castings among the gravel that smell nice and earthy like good soil, compost etc, then your system is most likely not anaerobic and those worm castings are doing your plants a favour. I’ve never had a major nutrient deficiency in the five years this system has been running and I put that down to the worms and their nutrient rich castings.

    Not all “muck” in an aquaponic GB is bad!... Don't be in a rush to completely clean out a GB, investigate first.


    francois likes this.
  2. Murray

    Murray Site Admin Staff Member

    Jun 4, 2009
    A good post Yabbies. Very interesting.
  3. Pamela Murry

    Pamela Murry New Member

    Jun 14, 2018
    United States
    Santa Clara
    True! Very interesting.

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