Whether from a lack of experience, or an abundance of neglect, some aquarists allow their reef tanks to reach a state of utter nastiness. Perhaps you’ve been there yourself… Where you once 
had a gorgeous, natural looking, sparkly clean system, there are now patches of filamentous and film algae, piles of detritus, clouds of suspended particulates and excesses of dissolved nitrate and phosphate. Physically and chemically, the environment is a total mess. Some hobbyists finally give up, pull the plug on the tank and walk away forever. Some take the relatively easy way out, flushing the systems clean, maybe even bleaching the rock, and starting again on a completely clean slate. The bravest of hobbyists might try to rehabilitate the system as it is. This latter response, while bold, is certainly not one that produces fast results. To the contrary, it could take several months to eradicate the bad algae, regrow the coralline algae, clean the substrate, etc. So, are you really committed to fixing this whole thing? Ready for the many water changes, many tests and hours of algae scraping? Ready to change your chemical filter media and rinse your mechanical media like never before? If so, then one potential piece of your remediation strategy might seem relatively easy: Add a huge wad of chaeto.
Planted refugia do all sorts of things to improve an aquarium environment, from stabilizing pH to creating biogenic habitat for pods. Chaeto (Chaetomorpha spp.) is a favored macroalgae for planted refugia, not just because it does most of these jobs quite well but also because it is very easily cultivated; that is, so long as extremely strong full-spectrum lighting is used.
Like most other green algae, chaeto requires intense illumination to grow properly. To illustrate this, we wouldn’t suggest using a light any less powerful than you would trust to grow healthy SPS corals. In order to grow a fat bed of chaeto, you might want to consider lighting the sides of the ‘fuge as well as the surface. Special reactors that illuminate the algal mass from multiple angles will be your best option if you’re trying to grow the most chaeto in the smallest space possible.
Chaeto as a living chemical filter
Perhaps the biggest reason aquarists add macroalgae to their systems (especially in planted refugia) is to control dissolved nutrients. And macros do indeed do this well–continuously and naturally. All macros. But chaeto in particular is prized for its ability to rapidly sequester nitrate and phosphate. This is because of its potentially explosive rate of growth.
Again, to achieve these high growth rates, you must provide extremely bright lighting. Also, you must frequently harvest small portions of your crop in order to keep the mass growing; as soon your chaeto fills the grow space and shades itself, it will stop growing. It might even start to die back, releasing nutrients back into the aquarium water as it decomposes! Whenever possible, harvest the oldest sections of the bed before the newest.
Be aware that chaeto’s capacity to remove some nutrients is limited if other nutrients become depleted. For example, it may cease to sequester nitrate if it uses up all available phosphate. It might stop taking up both nitrate and phosphate if it runs out of some vital trace nutrient such as iron. And so on… Thus, as soon as the chaeto has appeared to reach its limit in removing one or more nutrients, generous water changes are recommended; water exchange will not only remove the remaining excess nutrients, but also replenish essential trace elements!
Chaeto as a living mechanical filter
Though it is seldom mentioned (curiously), chaeto works as an amazing mechanical filter. Its rigid, wiry structure forms a meshy pad of sorts. These tangled bundles are very effective at trapping any suspended particulate matter that gets drawn into the ‘fuge.
Reactors work best for this because they force the water to flow through, rather than over, the mat. Still, so long as the water courses through the mat in just the right way, you’ll find (even in a “regular” refugium) that crazy amounts of solid wastes get stuck in the thick algal matrix.
This is significant for two reasons. First, it means that the main tank will look cleaner, since most of the gunk will concentrate in the ‘fuge. Also, it means that the amphipods and harpacticoid copepods will very easily find and consume it. So, not only will your pods have an ideal place to live (they love the sheltered inner space of a chaeto mat) but they will also have their food delivered right to them. Perfect!
To avoid shaking out too much of the entrapped detritus when harvesting, cut sections out with a sharp scissor instead of trying to rip it away in chunks.
Conclusion
Of course, refugium macroalgae alone can’t do all of your tank maintenance for you. But used in tandem with a robust clean-up crew that includes copepods, a consistent water change regimen and frequent servicing of filtration devices, chaeto can bring about significantly better water quality and a cleaner physical environment with less “bad” algae.
