15 Good Reasons to Have a Trickling Bio-Filter

Biological filters are the means by which people like you and I are able to grow our own fish.  In short……no bio-filters; no aquaponics!

Bio-filters are simply good places for nitrifying bacteria to live and play.  They provide the optimum environment for the bacteria that convert the potentially toxic wastes produced by the fish into organic plant food.

Bio-filters take many forms.  In a media-based recirculating aquaponcis system, the grow beds serve as bio-filters.  In a raft system, the grow tanks usually foster much of the nitrifying activity. 

The understated hero of backyard aquaculture (and aquaponics), however, is the humble trickling bio-filter.

Now, let me be clear right from the outset, I’m not suggesting that we all dispense with our media-based or raft growing systems. 

What I am saying is that trickling bio-filters will make any aquaponics system more resilient and more productive and that there are good sound reasons for having them…..including:

1.   They provide for much more nitrification than a grow bed of a similar size – simply because they have a continuous flow of water through them as distinct from the intermittent flow of a flood and drain grow bed.

2.  They weigh much less than a grow bed or grow tank.

3.  They enable the staged development of an aquaponics system.  The fish tank can be set up, cycled and the fish added……and then the growing systems can be added as time and resources permit.

4.  They are much cheaper to build than another  grow bed – good for those with a limited budget.

5.  They have a much smaller footprint than a grow bed – good for those with limited space.

Trickling bio-filters come all shapes and sizes

Trickling bio-filters come all shapes and sizes

6.  They will accommodate a wider choice of media.

7.  They offer greater portability – good for people who rent their homes or who are anticipating a move.

8.  They permit greater control over overnight temperature variations in an aquaponics system.  Some people shut their pumps down overnight to limit heat loss out of their system.  While I understand why they do this, it’s a risky practice. The inclusion of a trickling bio-filter into their water column will allow these people to disconnect their growing systems without prejudicing nitrification and oxygenation during the period of the day that it’s most needed.

A wide variety of media can be used.

A wide variety of media can be used.

 9.  They facilitate over-wintering more readily.  Fish can be moved indoors to avoid extreme weather….cold or hot.

10.  They are much easier to maintain than grow beds.

11.  They enable the separation of the fish tank from the growing systems in the event of disease or infestation in either fish or plants – which means that one can be treated without doing harm to the other.  

12.  They provide for more effective aeration – largely due to the continuous water flow.

13.  They are more convenient for ancillary small systems like fingerling system purge tanks or quarantine systems.

Trickling bio-filters can be used indoor or outside.

Trickling bio-filters can be used indoor or outside.

14.  They enable higher stocking densities – particularly when coupled with solids removal devices.

15.  They enable the integration of a wider range of growing systems – aquaponic or soil-based.

Given the advantages that they offer (and in the absence of any good reason for not having them), I strongly recommend that they be a feature of any aquaponics system.




  1. Steve Argent says

    Gidday Gary.
    Finally started preparing & setting up a 450 sq/mtr totally dedicated Aquaponics area at home.Like you Mate been very busy & now quite excited !!!!! Still make time to read your Blog & Forum regularly.
    Anyway Gary ,is it possible to run an Aquaponics Backyard System, indefinitly, with a good sized trickle biofilter ,swirl filter & clarifier filter & no grow beds?
    eg 1000 to 1500 litre Fish Tank + 100 litreT/ bioF + 200 litre S/F + 60litre C/F with a total fish load of 60 to 80 kgs.
    If not, what precautions would be necessary to enable long term operation of an ABS as above,( with no Grow Beds in this system)
    Thanks again for your time,
    Steve A

    • says

      Steve…..it sounds like you’ve caught the aquaponics bug. Regrettably, a cure requires long and protracted treatment and is often not successful.

      By definition, an aquaponics system comprises a recirculating aquaculture system and a hydroponic growing system. What you are proposing is a straight aquaponics recirculating system (RAS).

      While a RAS will certainly work, I would advise that you have at least one aquaponic growing system connected to a RAS. I suggest you read my earlier post on The Aquaponics Eco-System for an understanding of why this is a good thing.

      Once you’ve established a basic aquaponics system, you then have a choice about whether you attach more hydroponic growing systems……or soil-based options like wicking beds.

      I would certainly advise a more modest stocking density than the 60 to 80kg you’ve suggested. The system that you’ve described would be better suited to something in the range of 25 – 30kg depending on the species (and other factors – see Mythconception #1 – Stocking Densities) and assuming that everything was well designed and working well.

      If, for example, you decided to keep jade perch, you might decide to incorporate a duckweed tank into your system. This would turn your RAS into an aquaponics system (with the benefits of such), provide you with a modest bio-remediation effect and provide you with supplementary food for your fish.

      Obviously, a system of this type would be generating more nitrates than you would require for the duckweed so you could then set up some wicking beds and irrigate them using water from your system……which you would then replace with rainwater or dechlorinated mains water.

      A little system like this could be simple to set up and operate……..and would be resilient and productive.

    • says

      Brad…….the tanks are called mega bins and they are designed to transport and store horticultural produce. You should be able to buy them (or something similar) from handling systems suppliers. I live in Brisbane so I get mine from a company called People in Plastics.

  2. Steve A says

    Thanks again for your guidance,Gary.
    Its FINALLY sunk in.
    If there is no natural symbiosis between the fish ,bacteria,water & PLANTS + working in as many other natural intergrations + solids removal , as possible , into my system ,then Im only kidding myself on the long term viability & productivity of any Aquaponics Systems I will build now or build in the future!!!
    May I never be cured of my affliction,
    Steve A

  3. John says

    Can you explain – for us beginners- more about what a “Trickling Bio-Filter” actually is and how can one build one? You do a great job of explaining what they do, but I’m having a hard time visualizing how this would fit into my system. Are there any websites with some basic diagrams etc? Thanks,

    • says

      John…..you can build a trickling bio-filter from any plastic drum that is robust enough to contain the media that facilitates the colonisation of the bacteria. Insert a bulkhead fitting in the bottom of the drum. Water is pumped to the top of the drum and allowed to percolate down through the media before draining out of the bulkhead fitting back into the fish tank.

      The filter media can be any of the following……oyster shells, expanded clay pebbles, polystyrene beads, Matala mat or manufactured plastic media (Kaldnes, Bio-balls, PVC ribbon). There are dozens of different media suitable for use in tricking bio-filters.

      We’ve also made trickling bio-filters our of stacking plastic crates and small fibreglass tanks.

  4. SolidsBuildUp? says

    Hi Gary,
    Can you explain how you remove the solids from the filters?
    I like the idea, but can see how they might need frequant cleaning out as the fish poo starts to biuld up as the number and size of fish grow. ie, more fish, more feed, more poo, more cleaning.
    Also, what sort of volume of filter do you recommend for a given quantity/weight of fish?

    • says

      SolidsBuildUp?……a well-designed trickling bio-filter will not collect much in the way of solids. Since bio-filters work best if solids loads are minimised (something that many flood and drain aquaponicists ignore), I recommend the incorporation of solids removal devices upstream of the bio-filter.

      Having said that, I have a couple of small units that have the outlets located about 100mm from the base. This allows sediment to build up over time but it is easily removed by shutting off the flow to the bio-filter and, placing a bucket below the outlet, tipping the unit up so that the water that remains in the base of the bio-filter drains into the bucket. The spent sediment runs into the bucket. This takes less time to do than it takes for me to tell you about it.

      Calculating the size of trickling bio-filter for a given weight of fish is possible but the maths involved will challenge most people. I operate small backyard aquaponics systems, so I rely on testing for ammonia and nitrites (at the highest likely feeding rates in the worst climatic conditions) to indicate whether I have sufficient bio-filtration.

      Since I always have spare tanks (and bio-filters) on hand, I can spread the fish biomass across several tanks if need be.

      Over time, I’ve developed a reasonable understanding of how many fish of a given species and weight I can carry in a tank of a particular size…..at a given feed rate. You’ll know if you’ve got it right if your water tests indicate ammonia and nitrite levels that are within acceptable limits…..and if your fish are behaving like they should.

  5. SolidsBuildUp? says

    The buckets you use with oyster shells look very small relative to the volume of water in your fish tanks below them. What sort of ratio’s are you using there? Woudn’t have thought the maths to be quite “that” challenging????

    • says

      SolidsBuildUp……the “buckets” are 65 litre plastic drums……and the tank usually contains between 600 and 700 litres of water. I’ve reared up to 44 jade perch to plate size in one of these systems with periodic support from a cannister filter (when I’ve got a little too ambitious with the feed) and some modest water replacement (using the water for non-recirculating growing systems) so I know that’s possible.

      I suggest that this particular arrangement would be capable of supporting a similar load of silver perch or tilapia for those who are allowed to keep them). Many other species (eg…..trout, barramundi and murray cod), have a need for water of higher quality, so the stocking rate would need to be reduced.

      Of course, using larger drums would provide greater surface area for nitrification and would accommodate larger numbers of fish in greater comfort and safety.

      Here’s a link to a bio-filter specialist…….http://www.biofilters.com/websize.htm. I suggest that the process that they use for sizing a bio-filter would be beyond most backyard aquaponicists…..in terms of either their mathematical ability or interest.

      I figure that if I ensure that (by testing the water) the ammonia and nitrite levels are being managed effectively, I’ve got enough nitrification happening. Aeration takes care of itself……since there are few more effective ways to oxygenate water than to drop it percolate it through an air rich environment.

      What sizing method do you use?

  6. Dan Hofmann says

    Fantastic stuff. I am a newby to aquaponics but I have done a fair bit of research.I am planning an aquaponics system but my first project is to convert our pool into a natural pool and if it is feasible in the future, make it part of the aquaponics system. My wife is protective of her pool and dubious about my ideas so I need to make it so it has its own functioning system that’s as low maintenance as possible and here’s the crunch, as compact as possible. All the natural pools seem to work on vast reed beds with a square meterage equal to the pool itself. I just don’t want to give up that much space on our small urban block. I discovered biofilters for ponds but noone seemed to be swimming in them. Is there a reason for this? are the health concerns justified or can people just not see that its no more dangerous than swimming in a lake? I would love to be able to use an appropriate quantity of biofilters with a duckweed tank if necessary and if I could fit it into less than 5m2, I’d be a happy man. So, to cut a long story short, no fish, as little plants as possible, and a pool that two or three people can swim in every day for a week in the middle of summer and by no one all winter. The pool is above ground and 11m2 x1m deep with no overhead shading. I’m fully aware that this is not really your thing but I can’t find anyone else open minded enough or who’s ever really tried it.

  7. Dan Hofmann says

    I just discovered aquaponics hq and thought that might be a better place to post this so I’ve pasted it into aquaponics ideas. Looking forward to your response

  8. Steve A says

    Gidday Gary
    couldnt sleep,thought Id read, relax & ponder for abit.
    With regards to the oyster shell biofilters you have built & run, would it be fair to say that the o/shells would have some helpfull ,buffering effect on the system in general? If, whilst building my o/shell biofilter,I added acouple of well worn horseshoes or a couple of pieces of mild steel ,mid filter,would it ,in your experience, perhaps have enough beneficial effect to the general iron levels,to reduce or completely remove the need to add iron chelate to a system?

    • says

      Steve A……mild insomnia seems to be a common trait among aquaponicists. There’s always a big idea to ponder.

      While the oyster shells might have some useful buffering effect, I’ve never triailled it conclusively. Logic tells me that if a system is heading toward the acid end of the pH scale, the water would begin to dissolve the calcium in the shells. To what extent this happens (and whether it would be enough), however, I can’t be certain.

      Rusting iron will not add iron in the form that your plants need it so you’re stuck with the iron chelate. If you run your system at the low end of the pH scale (6.0 to 6.5) iron, and a number of other nutrients) become more readily available to your plants – than would be the case at higher pH.

  9. says

    biofiltration is all about offering growing surface and oxygen to beneficial bacteria
    in that aspect all my calculations show plastic polypropylene or polyethylene beads (cheap to buy, the ones used for extruding plastic toys and utensils) to offer the most surface next to sand
    with the advantage that these beads are slightly less dense than water, which is why they just float, contrary to sand which sinks.
    so sand needs a lot more more energy to be kept in movement so as to expose the most of it’s surface to the bacteria
    trickling filters with whatever material are fine, but TMHO take up far too much space: a comparable upflow bead filter has a very small footprint
    it can be combined with the clarifier. Done some tests on this.
    trickling filters do clog over time and then are very difficult to clean
    it must be possible to design an upflow bead filter that needs very little maintenance, say 95% self cleaning
    would like some help with this

    • says

      Frank……when it comes to nitrification and solids capture, fluidised bed sand filters are (theoretically) the way to go…..largely because of the huge surface area of the sand. The problem is that, in an aquaponics system, they appear to clog to the point where water flow is seriously restricted.

      Bead filters seem to be almost as good as sand while being less affected by the clogging issue…….and they are easier to clean. The biggest issue with them (for my purposes) is their cost.

      I’ve used trickling bio-filters for several years without the clogging issue. This may be because I’ve used whole oyster shells as the media. While they don’t offer the surface area of smaller media, they have proven adequate for my purposes. Aeration (a large part of the reason that I use them) is excellent and the price (free for the taking) is right, too.

      Recent trickle filter experimentation with polystyrene beads (like they use in beanbag chairs) revealed that, while they offered much greater surface area than the oyster shells, they did have a tendency toward clogging and subsequent channelling of the water.

      I think bead filters are the way to go for the reasons you’ve identified. For me, their big attraction is their capacity for nitrification and their ability to remove fine solids. I quite like the idea of the bubble bead filter…..and I don’t find the idea of a few minutes of maintenance each day too onerous (given the other benefits they provide).

      Have you considered using an air pump……..to churn the water and beads around so that they let go of the attached solids……as a relatively quick and easy cleaning option?

      You could then just dump the dirty water and either use it directly on your soil-based gardens or expose it to aerobic digestion so that the nutrient-rich liquor could be decanted and put back into the aquaponics system while putting the sludge onto the compost heap.

  10. says

    I have serious doubts about the use of air pumps: according to my calculations, their efficiency is even worse than that of the pumps we use. Fundamentalists have argued with me on this subject on all forums but not one of them has come up with proof of my wrong, not even close. So TMHO better use a pump than an air pump, or mechanical means (mixer, rotating brush) to clean the beads.
    Polypropylene (PP) or Polyethylene (PE) beads are cheap, and if you go to a truck cleaning station where they clean bulk trucks you can often get some of what was left in the truck. Or go to a plastics extrusion company: they may have some leftovers that don’t meet their quality standards.
    You don’t need a pressure vessel to build an upflow filter, it can be athmospheric. You just need a screen to hold the beads down.

    • says

      Frank……..while I agree with your observations about air pumps, they get used for the same reason that we use the relatively inefficient submersible water pumps……the sheer convenience.

      I think the popularity of air pumps is probably more the consequence of ignorance about how water takes up oxygen.

  11. says

    the performance of air pumps at pumping is extremely poor, they waste 99% of energy, my calculations prove it

    people are fooled by the fact that building an airlift pump is very simple
    To design and build a performing airlift pump is extremely difficult
    very little literature is available, very little design parameters
    most is trial and error, mostly error

    seeing bubbles rise in the water creates the optical illusion that the water is efficiently aerated, everybody would fall for it

    we need only 1 m of water level in the fish tank, say maximum 1.20 m head in the whole system, all pumping above this is a waste of energy
    if you pump your water overhead, say to 2.40 m, you waste 100% of energy

    the performance of air pumps as a means to oxygenate the water in aquaponics systems (or in aquariums) is simply another widespread myth:
    air bubbles only perform well at oxygenation in depths over 5 m as there the water pressure on the bubbles ensures good gas exchange
    but then, you need 5 times more energy to pump the air to a depth of 5 m


  12. says

    Frank……I was actually suggesting the use of an air pump to churn the beads when cleaning a bead filter. In this situation, they only run for a few minutes.

    Of course, you could use a water pump for the same purpose.

    • says

      Peter……air stones in a trickling filter would provide minimal benefit but you could use a small fan (like those used on computer power supplies) to force a draft of air upwards through the media. Of course, such a fan should be positioned and installed in such a way that there was no electrical hazard.

  13. Greg Holburt says

    Hi Gary, we have built a 3,000 litre system based on the 3 Nally bins for fish, one 200 litre swirl filter and a Nally bin bio filter. I need some bio balls or similar, any suggestions? The water has been in circulation for a few days and has gone a little cloudy? PH is around 7, the insides of the tanks feel slimy, rain water was used to fill the system initially, no fish yet, any thoughts?


    • says

      Greg……Your system sounds great. If you haven’t already done so, I encourage you to register on Aquaponics HQ forum so you can post some photos. I’d love to see them and I’m sure our members would, too.

      My own Mega Bin System comprises three tanks, a 100 litre swirl/packed media filter and a 200 litre moving bed bio-filter/sump.

      Having a clear picture of how your system is set up will also enable me to provide more effective advice.

      The conditions in your tanks are normal……I’d suggest that your rainwater has some nutrients in it and the slimy surfaces are the result of bacterial activity.

      How are you planning to cycle your system?


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