and to add to dan's response, beyond the ats, one of the latest things being touted is the Algae Remediation Illuminated Device or ARID. sounds complicated, but basically they are a reactor like we use for carbon, biopellets and the like, surrounded or with a central core of lights favoring the red end of the spectrum, in which chaeto is grown rapidly. the purpose is the same as a refugium and an ats, the exportation of excess nutrients by growing and harvesting macroalgae. i've been fighting algae on my main tank despite refugia full of macroalgae, so i'm going to be installing one of these and see how it helps.