Homemade Underwater Filters

Underwater filters are an easy way to add extra filtration to your aquarium system. There are three types of filtration you need to keep your aquarium healthy: mechanical, chemical and biological. Homemade underwater filters can help with all three by providing a place to add filter media (mechanical filtration), granular activated carbon (chemical filtration), and beneficial bacteria (biological filtration).

  1. The Pump

    • To make an underwater filter, you need a small submersible water pump called a powerhead. Bigger is not necessarily better. If you use granular activated carbon, the material filters better if left in contact with the water longer, which means you want to use a smaller powerhead. Purchase a powerhead, which you can fit a piece of vinyl hose to the outlet, so you an build the filter.

    Filter Case

    • The case of the filter can be made of any watertight material. Clean plastic water bottles work well, as do 2-liter soda bottles. You can also use plastic food storage containers. Whatever you decide to use, clean it thoroughly without using any soap. Use rubbing alcohol to remove stains, grease and glue. If you need to seal seams, use aquarium-safe silicone.

    Building the Unit

    • Attach a vinyl tube to the outlet of the powerhead and insert the other end into the container. The tighter the fit, the better it will seal and prevent water from leaking out; if it is tight, you do not have to use silicone to seal it completely. Cut off the other end of the container so you can add your filter material. Reattach with a rubber band or string and cut holes in it to allow the water to flow through.

    How to Add the Filter Media

    • Two good choices for filter media are nylon or cotton mesh and granular activated carbon. Add these so the water flows through the mesh first, then through the carbon; this helps extend the life of the carbon. You only need about 1 tbsp. of activated carbon per gallon, but you can add more so you do not have to replace it as often. Keep an eye on the mesh to make sure it does not clog. Replace if it starts to fill with debris. A clogged filter causes backpressure on the powerhead and may burn out the pump.