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I've been studying aquaponics for a couple of weeks now and even though I learn new things each day, I find myself returning to an unanswered question.  If the whole idea in aquaponics is to use fish poo for nutrients, then why do we filter it out in DWC/Raft systems?  I know the general answer to that is to keep the roots of the plants clean.  But if we filter it out to keep the poo from building up on plant roots, where do the nutrients come from to feed the plants?

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Hey, to answer your question, the reason is that they use the filters to create a space to filter the poo and create an area for the bacteria cultures to grow on the filter surfaces, so they can convert the fish waste to a use-able form or nutrient. If the filters where not there, the poo would be allowed to flow freely into the raft area. If this would occur, there would be a possibility to build up the solid waste and create an anaerobic zone, which would then start to kill the system and deplete the oxygen in the system.

I don't flush my poo, nor do I flush my fish poo! http://www.humanurehandbook.com/

The only systems that remove fish poo are the ones that have some means of solids removal or settlement that they have to clean out regularly.  Now those are necessary when you have a heavy fish load with not enough filtration/aeration to handle the solids.  Most media bed systems with the 2:1 grow bed to fish tank ratio don't need to remove solids unless they are grossly over stocking and over feeding the fish.

This is the way i see it working. The fish poo in the water which contains ammonia, The ammonia is suspended and moves freely in the water. The poo settles out and begins to build up in the gravel beds or filters. By removing the solid matter, you keep the level at a minimum so the air can get to the roots of your plants. It also keeps the bacteria happy and prevents your system from going septic. And if the poo is allowed to build up it will eventually get back to your fish tank and your fish will not like breathing all that.

Your gravel bed containes bacteria that eats the ammonia, and they poo nitrite, that when aerated, turns into nitrate. This is what the plant uses. If your poo is left in the gravel then the aeration is not as effective.

So i guess to answer your question, we filter out the poo because it is not needed by the fish,plants,or bacteria. But it is great in a compost pile for use in traditional gardening.

Hope this helps and remember my first sentence (This is the way i see it working)

In flood and drain media beds and gravel beds, the solids or fish poo that collects there gets mineralized by bacteria and worms and isn't a problem as long as there is plenty of gravel bed to support the added load on the system.  By removing those solids from the system you are essentially requiring that you need to feed more fish to grow the same amount of plants.  So as far as I'm concerned the plants and bacteria need the fish poo because I grow more plants with less fish.

I only recommend removing the solids if the bio-filter is overloaded or too small for the amount of fish.

I've found that with twice as much gravel bed as I have fish tank, I can stock that fish tank pretty darn heavily and still have more than enough gravel bed to handle the solids without negatively impacting the plants or bacteria, indeed positively impacting the worm population with is generally good for both bacteria and plants.

Thanks TCLynx, I totally forgot about the worms. I'm still putting my system together, so i will make my beds bigger than needed, cause i'm not sure of my fish load yet and don't want to run out of bio- filter.

Yesterday i spoke with my brother inlaw, who is into saltwater. I asked him about turning the water over and pump sizing. He said that its not really important that the water is turned over every hour, but it is more important that the water that is turned over gets treated properly and returned to the tank very clean. He has a 300 gallon aquarium and he runs his effluent through a 10 gallon tank that has a grow light and plants. His NO3 returning to the big tank is near 0. He has no plants in the big tank, only corral. Is this ideal?

Saltwater aquarium operation is different.

And most aquarium are not as heavily stocked as some aquaponics systems.

There are of course exceptions to every rule but.  If you have twice as much grow bed as you have fish tank volume (and you don't go stocking your sump with fish.) you should have enough grow bed to handle the solids from the max stocking of fish you can do without needing to use pure oxygen in your aeration.  Now I NEVER recommend some one starting a new system use the max amount of fish.  I'd start off with the 1 fish per cubic foot of media or that is about 1 fish per 7.5 gallons of media bed.  And if you cycle up fishless you should have enough nutrient to get you going even with starting out with small fish in the beginning.  Mind you that one fish per cubic foot is 1 fish that you only grow out to about 1 lb.  If you are going to grow something like channel catfish, they get bigger so you stock less of them and then you start harvesting some kinda small to let the rest get even bigger.

Now some people like to remove the poop so that they can run with less grow beds or they are doing raft culture which requires cleaning the water before sending it to the exposed plant roots.  Or some people like to remove the poop because they like to use it as fertilizer in their soil beds.  So what you do will affect where you put your labor.  I don't enjoy cleaning out settling tanks so I would rather let media beds take care of that instead.

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