3" of sand is too shallow for any DSB
benefits and there's not a lot of area in a sump/fuge for that anyway. Generally a fuge should have much slower water flow, so it should be separate compartment from the sump fed by a small pump. That's the theory anyway.
I use the last compartment of my sump to hold some lr
rubble, sand and chaeto. When I was running bio-pellets, GFO
my chaeto started to disappear as the nutrient level went down. Before that the caulpera out competed the chaeto for nutrients and grew like crazy.
When between having major back issues and being too busy at work to give my tank proper maintenance (changing GFO
/ pellets) I started getting algae problems in the tank and added some chaeto back to the sump. I'm now pruning it regularly. But I also did my semi-annual maintenance over New Years and now have new bio-pellets, GFO
running. I expect my chaeto may do another disappearing act.
So, if you're going to add bio-pellets and your already running (and changing) GFO
regularly, dump the fuge. Otherwise, give it some more time and you may start to see some regular growth.
BTW, caulpera as I recall should be on a 24/7 lighting schedule. Having it on an opposite schedule is what triggers it to go sexual.