When the return pump is off (water change, power outage, whatever), the water level in the sump rises, obviously. One annoying downside is that stuff (macroalgae, whatever) then has a means of moving from, say, the refugium section to the return pump section. Sure, it can be manually moved back, but it seems like there could be a way to stop that. Maybe a rigid egg crate that goes to the top. Maybe some sort of floating bar that pulls up a screen. Anyone doing anything cool for this problem?