I’ve experienced this so many times. My impression is that if/when you go to an area of no cells and bleach that due to diffusion, the molecules in the media that are responsible for the ‘background glow’ will simply diffuse back in (anecdote - the background glow does seem sometimes decrease somewhat over time in long timelapse runs, but this isn’t a good way to address the problem). A better solution is to buy some phenol red free medium, which is readily available for most standard formulations, this removes a lot of the glow. If you’re needing to also remove riboflavin and tryptophan (I think), you can get this medium, but it is traditionally a lot more expensive (I was once quoted $500/500 ml bottle). I’m not too familiar with the commercial background suppressors, but it sounds interesting. Depending on how long you plan to image, there are also products like “Live Cell Imaging Solution”, which is an optimized buffer where cells can be out for 4 hours or so, this has zero fluorescence. Good luck! Based on the image you provide, which look like nuclei at 10X in a wound assay (could be wrong), I’d say phenol-red free is the way to go.