hi Have you tested for silicates/silica at all?
My PO4 were fine it was my silica that was slightly high. Do you have a TDS meter on your RO/DI unit?
How deep is your sand bed? I recently found a few buckets of Instant Ocean and Reef Crystals with high silica. A water change and turning the lights opff only help as a temp fix. You really need to get to the root cause of it all. I would recommend getting a silica test kit, if you do not have one yet and see what you get. If possible get a complete water test done. Then start tracing where the issue might be coming from. Slowly make changes and wait a week between changes to see if you get any results. This is nothing that will be solved overnight and we have all battled it at one point or another. The downside is everyone's issue was most likely caused by different things including all the way back to the quality of water coming out of your tap into your ro/di unit.
Goodluck,
Andy