How to Build a Diatom Filter
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Water-polishing filters remove algae, diatoms, bacteria and other contaminants from aquarium water. These high-powered filters were traditionally made using diatomaceous earth (DE) to coat filters and produce a micron-fine screen to polish water.
However, DE particles are extremely sharp and can cause serious injury to fish gills, and DE-based diatom filters are expensive. Modern micron filter cartridges function as well or better than DE in fine diatom filters. You can make your own diatom filter in less than an hour with materials from your local aquarium supply and hardware store.
Cut a 1/2-inch length and a 1 1/2-inch length of 3/4-inch-diameter PVC pipe.
Attach PVC end cap to the 1 1/2-inch length of 3/4-inch-diameter PVC pipe by twisting tightly into place.
Insert the capped length of 3/4-inch diameter PVC pipe into the hole on one end of the canister-style filter cartridge. It should fit snugly.
Place the 1/2-inch length of 3/4-inch-diameter PVC pipe over the standpipe inlet of the powerhead pump.
Insert the PVC-ringed standpipe inlet of the powerhead pump into the open hole on the end of the canister-style filter cartridge.
Place the assembled filter into the corner of the aquarium so the capped PVC pipe rests on the bottom. Make sure the powerhead is completely submerged before you turn on the pump.