These type of filters are commonly used in "high end" aquariums. They can handle a very high bio load for their size. When I was into aquariums, I had a shitload of african cichlids in a 200 gallon tank and used one of these as my main ammonia scrubber with an active carbon polisher.
Disclaimer: These types of filters are very low maintenance, but if you lose power for more than an hour or so you are going to have to throw away your media (sand) and start over. The reason for this is because the bacteria will suffocate quickly and the filter will stagnate, the sand will start to stick together and it will no longer work.
Since we keep our water temps warm (relatively speaking) at around 70f or higher, it wont take long to colonize the filter with bacteria (3-7 days). The reason that this type of filter works so well is that the surface area of sand is ridiculously high for the volume that it takes up.
Once the filter is colonized we should be able to stop adding beneficial bacteria to our nutrient mix.
All we are doing is giving the bacteria that we are already adding a place to live permanently, and providing them with food (plant matter).
I don't know if this will work with DTW (drain to waste), because normally you would find the highest concentration of bacteria colonies on the roots of the plant because they eat dead plant matter and organic acids, and fix nutrients so they're usable by plants, what we are doing here is moving a very large bacteria colony to the recirculating reservoir and providing them an ideal environment to grow so they just keep doing their job undisturbed.
End of disclaimer
Now, all that a fluidized bed filter is, is a container that directs water flow up through sand which makes it fluidized.
There are a lot of different ways you could do this with parts laying around and there are a ton of DIY videos on youtube for "fluidized bed filters".
The way I want to do it still costs less than $50 in parts if you buy everything brand new.
Make sure that you sterilize everything first by using bleach, and boiling the sand. We want the "good bacteria" we are about to introduce to not have very much competition.
I know this should go without saying but don't try and do this if your reservoir and plants aren't healthy. You'll just give the bad bacteria a huge foothold. The inverse of this is true as well, as long as this is colonized with good bacteria your recirculating system will have a muuuuuuuuch lower chance of developing bad bacteria.
Parts list to make your own:
Silica sand, 100% pure.
https://www.amazon.com/Lifegard-Aquatics-Filter-3-Pound-Bottle/dp/B00025666U/ref=sr_1_1?ie=UTF8&qid=1494522313&sr=8-1&keywords=fluidized+filter+media
It doesn't matter where you get the sand, what does matter is that you wash and sterilize it before you introduce it to your bacteria, and that it is 100% sand and nothing else.
Some way to adjust the water flow (on the outlet side), a ball valve or adjustable pump. Whatever you have laying around. What you want is for all of the sand to be moving in your filter, and none of it to really settle because that's when you can run into problems. You also don't want too much flow or the filter will spit the sand out. You also don't want to over fill it with sand or it will spit sand out. I used to start with a little less than 1/3 full of sand, you can add more once you get things adjusted.
This is the media cartridge, where the sand goes:
https://www.amazon.com/gp/product/B01CFOFW3S/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER
What you want to do is put the foam piece that comes with it on the bottom, then put sand in, and then put some "filter floss" in the top to minimize the chances of sand getting where it shouldn't be.
Now you need a housing to put that cartridge in, any housing that will accept a 2.5"x10" media cartridge will do.
https://www.amazon.com/Hydronix-HF2-10CLWH12-Clear-Body-White/dp/B00D04CHRM/ref=sr_1_3?ie=UTF8&qid=1494523759&sr=8-3&keywords=1/2+filter+housing
The inlet and outlet size aren't super critical, but remember you want to pump a lot of water through that sand because if the sand is allowed to settle it wont work right. So anything smaller than 1/2" I wouldn't recommend.
As far as a pump for it I would go no smaller than 200gph for a filter of this size.
Hook your pump up to the inlet side. And your ball valve or to the outlet side, crack open the ball valve and plug in the pump. Adjust the ball valve until all of the sand is moving. If the sand isn't getting close to the top you can add more, and readjust the flow.
Now, a filter this size is good to filter around 100 gallons of recirculating water. Dstroy how is that possible? Well, because sand has a really high surface area and we are giving the water that we are pumping through it maximum contact with our filter media.
Now, hopefully we can stop buying so much mammoth p.
This might also work for people who brew organic tea, as long as they have decent filtration on the pump inlet so large particles don't make their way into the sand. All of those bacteria that they are trying to get to grow will stay alive as long as the pump still works and it will probably take much less time to brew tea because you already have a huge bacteria colony to kick start it.
good thing they sent extras.
