The tank has seen little change in 25 years. Jaubert Plenum was designed to use facultative bacteria in a low oxygen environment to perform denitrification biochemistry. Substrate was 6” deep with false bottom making a Plenum under substrate. Plenum performed well until the introduction of Melanarious Wrasse to control Red Planaria, which unknown to me eliminated my detrivore crew. With a 6” dsb with 25 years of detritus, I made the major decision to vacum sandbed down to < 2”. After removing several hundred pounds of substrate while syphoning out over 500G of water over a two week time period, I now have a thriving mixed garden with ornamental sponges and ornamental macro along with numerous other filter feeders including flame scallops and numerous fans & feathers that I have no clue to the name, but I consider it a healthy bioindicator. So, water flow from top to refugium goes into rock rubble first chamber (wet/dry) where gas is exchanged and detritus is broken up and settles into large second compartment with mud. In 25 years, I have never replaced mud. The depth in mud filter has increased in depth from .75” to 1.5”. It felt spongy to the touch and was crawling with worms. When I converted from macro refugium to cryptic refugium, I turned out the lights, composted tomatoes, added eggcrate in mud bed so rock was clear of mud, seeded cryptic sponges and have been very pleased. The flow thru this refugium is much higher than recommended by Steve Tyree. Flow in my tank is supplied in tank with one HOB at 400GPHR and one Cannistar filter at 300GPHR. Flow from refugium is 500GPH. Lighting is supplied by three 4’ BMI 108W bars with a color spectrum of 12K. Fish include: Blue Damsels(7) Flame Angel Coral Beauty BiColor Dottyback Green Mandarin Blue Chromis Banggi Cardinalfish Melanarious Wrasse Sailfin Blenni Sponges include: https://www.live-plants.com/orangesponge.htm https://www.live-plants.com/elephantearsponge.htm https://www.live-plants.com/yellowball.htm Softies: GSP Yellow Colonial Anemone Xenia Green Sinularia Cabbage Leather Red Mushroom Ornamental Macro Bortacladia (Red Grapes) Chemical filtration is seldom used, but due to some cynobacteria, I am using phosphate resin and GAC.