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DIY Fluidised Sand Bed Filter

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I had this info posted on another forum a couple of years ago but I've been getting a few PM's about FSB filters here recently so I thought I would do a copy + paste here too.

Fluidised bed filters can handle huge amounts of bio-load because they culture beneficial bacteria at a young active growth stage. They can maintain young bacteria cultures because fluid bed filters are constantly moving which causes the filter media to rub and clean against itself. This scours off existing bacteria, especially the mature inefficient bacteria. However sand granules are irregular shaped so there is always some beneficial bacteria retained and it is constantly trying to re-colonise all the new exposed surfaces. It's this new young re-growth bacteria that is the most active and effective for biological filtration. If you have trickle filters or sumps, you will already know about all the mulm buildup from the dead bacteria. This coats the trickle media reducing its effectiveness, lowering gaseous exchange + nutrients available to beneficial bacteria and greatly increases filter maintenance. The other big benefit of a fluid bed filter (for me at least) is these filters do not do any mechanical filtration whatsoever so there is never any cleaning to be done. Even the smallest particle of dirt will work its way through the FSB filter eventually. Fluid bed filters will handle large sudden bio-loads due to the dynamic nature of its bacterial culture and consequently are one of the fastest filters to recover from an outage. This is good for me as I am constantly adding large fish and/or large numbers of young fish between systems from one week to the next. As I wanted extra emergency filtration, this type of filter was the obvious choice for me. I run this as supplemental biological filtration because its a cheap effective safeguard.

Rather than go into all the ins and outs of fluid bed filters Vs other types of filtration, I will start straight onto my build. There is plenty of technical information online about fluid bed filters for those unfamiliar with them or please feel free to ask questions as I go. This example is a fluid sand bed filter but there are other types of fluid filters too. Obviously not all filtration types are equal so you need to custom filtration to your situation. I'd prefer to answer questions than re-hash common knowledge to people. The only thing I would stress is a fluid bed filter should be regarded as a supplemental filter when starting it off. Young active bacteria are also the most oxygen demanding too so you may need to incorporate some re-gassing safeguards if returning filtered water straight back to your fish.

The other important aspect for me is the cost of filtration. Although I have a lot of tanks, I try to reduce setup and operational costs. Luckily you don't need to spend a lot to get good results if you know what objectives you are trying to meet. I found these 45 L bucket/bins as a clearance item for only $10 each. As I was making 5 of these filters, the cost saving was beneficial. Almost any container will do but these were good volume, tall & narrow, cylindrical and slightly conical towards the bottom. Most people prefer to use glass or acrylic tube (or PVC pipe with a clear viewing window) as a filter chamber so they can monitor the fluidisation. Its very hypnotic! However, since I am on a shoestring budget and I would be monitoring these filters from above anyway, I decided not to worry about side views. I opted for open top filter over a sealed unit for ease of access and reduced costs so it was important the water level in my filter be above the water level of the fishtank.

Comments are above each photo:

A quick visual measure to mark where I would be drilling. The line is the top level of my tank so all holes needed to be above this. Only two holes needed for an entry and exit. Note that the exit drain hole (left) is lower than my entry hole. Important for power outages.


I used a 32mm circular holesaw to accept 25mm PVC pipe fittings. I used standard 25mm PVC joins and a rubber washer as bulkhead/tank fittings. There isn't much pressure since its an open top filter.


Because the bucket was a little too wide to keep all the sand in constant motion, I decided to add a bell onto the bottom of my input 25mm pipe. This was a 2L plastic bottle to spread the area of sand stirred up. Sand is so cheap that you don't actually need all of the media to be used but I wanted to eliminate potential dead spots as I am not going to be cleaning these filters ever. Actually I really wanted to use a glass bottle for a bell but I couldn't find a glass bottle wide enough in my shed. I heated the pipe for a snug fit onto the plastic bottle then cut off the bottom and removed the label.


The practically finished product below. Just needs sand added next to finish off. There is not much to it which is appealing to me. Simple designs have less to fail and economical to build. The drain hole is the small T piece near bottom of this picture. I used a sideways T on inside as a safeguard against clogging. The bottom part of this T is the main drain path and the top acts as a breather and emergency overflow if the bottom ever clogs. IMPORTANT to make sure the emergency overflow is below the lip of your bucket!! I have enough experience to know one 25mm drain hole would be sufficient for the water flow I will be using. If your unsure, feel free to add a secondary emergency drain slightly higher up again or enlarge the main drain hole.

The input pipe now has the spreader bell (plastic bottle) installed at the bottom to help stir sand. Because I am using sand as my filter media, I wanted to ensure none would escape from the filter into my pump tank by back siphon because sand and pumps do not mix very well. You could take some precautionary measures like raising your pump off the floor or adding a sand trap but instead, I have safeguarded against a back siphon (during power outages) by adding a breather tube onto my input pipe. Its the really tall central pipe. I could have used an check valve to do this but simplicity is my mantra. The breather tube does not need to be this tall but this was the test version. None of the pipes are glued since there isn't much pressure involved.


This is my filter just filled and running. I had it on maximum flow to make sure the emergency overflow could cope so thats why water is draining down the top branch of T drain. I backed this flow rate down later as the sand was stirring well even at half speed. Yes I was too lazy to wash the sand but I know it will self clean in a few hours. Its on a fairly large system and the influx of some dirty water/sand may trigger some spawns.


This is a side photo of it running in test mode. There is a 1200LPH powerhead (20 watts) running this unit. It has 25Kg of sand on a 12,000L system. The sand filter will be able to handle over 30kg of fish by itself when it is fully active. I could post a video of the sand fluidising but youtube is full of similar items. After a few days of testing, I did shortened the breather tube and place the filter right under the stand out of the way. There is no maintenance required so it could be tucked away.


This filter currently maintaining 60 tanks - 3' x 2' x 15" and the other similar sized filters on larger systems.


Thanks for reading


Edited by aquaholic99
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