Do you have a Fuge grow light that you like? Would you mind sharing a picture of your sump setup? I have a new biocube32 (since 10/31) and have stable, but elevated nitrates (5-10ppm) and phosphates (0.08-0.12ppm RedSea kit for both) and lots of diatoms and algae with no improvement after adding chemipure blue a couple of weeks ago. I do weekly 20% water changes. I haven’t added any new CUC and have 4 fish so my bio load is stable. I would like to go without a skimmer (and would like to leave my biocube32 lid intact for now). Your setup sounds great.I've had my Biocube 32 up and running for 7 months.
I've never ran a skimmer and instead have the intank media basket (running poly and chemi pure blue) and an intank fuge basket with a grow light that comes on at night. I do monthly 10% water changes. I've isolated the chaeto to the bottom of the fuge basket (where flow is highest) by siliconing plastic cross-stitch material about 7 inches from the bottom. I then blacked out the rest of the clear glass of the back compartment with black vinyl decal material (to avoid the light shining anywhere but the chaeto).
The chaeto grows very fast this way and I have never tested above trace for phosphate and nitrates.
I have been able to keep parameters very stable during this time with a lot of SPS growing well. Just letting you know the option is there if you choose to go skimmer-less!