What is a Wet/Dry Filter?
A wet/dry filter, also known as a trickle filter, provides mechanical, biological and optionally chemical filtration for an aquarium. The wet/dry filter is the preferred filter for many aquarists because of the handy sump it contains. A wet/dry filter sits under the tank, normally inside the aquarium stand. There are many designs, but basically the water is drawn down from the aquarium where it enters a large rectangular container. The water first passes through a cleaning medium like a foam pad to pull particulate matter. It may then pass over chemical resins or activated carbon to adsorb organic pollutants. Next it enters a drip tray where it drops through air to gain maximum aeration before falling on to filter media below that is designed to house bacterial colonies that make up the biological filter. Once the water passes over the biological material, it enters a sump where it collects in a bath. A pump is placed in the bottom of the sump, and the water is returned to the tank at
Normal biological filter media (say, BioMax rings in a Fluval 404 canister) are submerged. Water flows over them and their bacteria break down ammonia to nitrite to nitrate. Cool. But their bacterial activity is somewhat limited by the low oxygen levels present in water. Oxygen is poorly soluble in water. But there’s a LOT of oxygen in air (21%). So, how can we bring high oxygen to aquatic bacteria? A wet/dry filter is wet but not dry. It keeps the media doused with a thin film of water, but not prolonged deep submersion. Oxygen diffuses rapidly across the thin layer of water covering the bacteria, who stay wet (and alive) but have more oxygen to use. In other words, a smaller amount of biomedia can handle a larger waste load faster than it could in a traditional constantly submersed setup (like the Fluval canister). One post I read in a fish forum said a wet/dry can break down ammonia & nitrite up to ~ 70% faster than a normal canister filter, but emphasized that wet/dry filters are m
