Fluidized bed filters use sand suspended in water to purify aquarium water. Water running through a sand bed suspends the sand particles, creating a relatively huge surface area-to-volume ratio, encouraging the growth of beneficial bacteria that break down fish waste. Despite their utility, fluidized bed filters have a relatively simple design, lending themselves to DIY versions.
You can get most of the supplies you need to build a working fluidized bed filter from the hardware store and a grocery store. Specifically, you will need several cups of playground sand, bulkhead fittings you can purchase online, a tall water container like a nalgene bottle, vinyl tubing or PVC that can fit onto your bulkhead fittings, and an aquarium pump rated to move at least five times the volume of your aquarium per hour. Since this project is very customizable, you will probably need to experiment with the specifics, such as the diameter of your bulkhead fittings. Pick your bulkhead fittings and tubing based on the outlets of your pump. A 5-gallon bucket can help with assembly.
Drilling and Plumbing
You will need to drill two holes in the lid to your bottle. Make sure you leave enough room between both the bottles and the threads of the bottle. You need to leave enough room to accommodate the outer diameter of the bulkhead fittings. It's very important you can screw the lid shut tightly. Teflon plumber's tape will help seal the joint, as will the rubber or silicone flanges that come with bulkheads. One tubing will be the inlet and the other will be the outlet. Cut three lengths of flexible tubing or PVC pipe. Two will have to be long enough to go from the aquarium to the filter, and one will have to be long enough to go from the bottom of the lid to the bottom of the filter. If you use vinyl tubing, the tubing ends will just slip over the bulkhead fittings.
Once you finish your drilling, cutting and other plumbing, assembly does not take long. First, attach all your tubing to your bulkhead fittings. One tube should go from the aquarium pump to the filter inlet. Attach the shortest of the three tubes to the inner side of the inlet bulkhead fitting. Then, attach a length of tubing to the outlet fitting.
Next, rinse your sand. To do that, put it in a bowl, and fill with water. Stir the sand and pour off the water. Repeat until the water you pour off is no longer cloudy. Then, fill your 5-gallon bucket halfway. This way you can submerge your pump, the inlet/outlet tubes and the container. Once you fill the container, dump your sand into the container and screw the lid on tightly. You may need to experiment with the exact amount of sand you can add.
You need to test everything in a bucket or a tub to make sure none of the joint leak. Never use a glass or flimsy container, since the pump could cause it to break under pressure. You also need to make sure the pump and outlet tubes are anchored to avoid spraying water everywhere. Ideally, you will want to set up your filter in a sump in case of leaks.