Media reactors are great for tossing in your favorite aquarium filter media to quickly filter the water in your aquarium. They can house multiple types of media, can be used in an almost unending number of configurations, and are rarely all that expensive to set up or maintain. Despite their numerous benefits, however, there are some drawbacks. While most of these might be centered around leaking fittings, extra costs, and things of that nature, we’ll focus on a far more subtle one. When running granular ferric oxide (GFO) for phosphate reduction, it’s easy to just let the water flow through freely. Unfortunately, this causes the media to tumble, which leads to crumbling of the individual grains. When this happens, small particles of the GFO will get past the filter pads within the media reactor and coat your corals in a fine brown dust. This could also happen, to an extent, with carbon media, so it is best to slow the flow through the reactors with the use of valves.