Scott did you ever try/consider cutting out the refugium to free up some extra volume in that lower sump?

I like this idea.

It's not backflow through the pumps, its the actual water volume from the display tank that is above the weir teeth draining into the sumps.When the top sump return pump is running, it raises the level in the main tank by ~1", the lower pump replenishes the top sump and makes up for any discrepancies in water height (usually due to evaporation). I already have a GF wye on the lower sump pump, so nothing drains back through it.

By my calculations that would only gain me 1.24g of volume.

Your refugium appears to be much bigger than that.

Yes but a refugium by nature is already filled with water. So say I cut out the refugium baffle, making the pump chamber that much larger. All I will gain for volume is about an extra 2" in height over a 14" x 12" (minus the emergency overflow 5" x 5"). My calculations come to 1.24g of water. Say I can lower the water another couple inches max!, I'd only gain another 2.6g and would likely start drawing in air vortices and blowing microbubbles everywhere. Does my math make sense to you guys or are you thinking of something I am not?

I still think my only option is a new sump.

Ok, so if it's not the back flow, then the only recommendations is a bigger sump. The bigger the better. :-)

I had to go with a 180 gallon sumo and only run 8 inches of water in it, tank is 30 feet away 2 inch drain lines 30 feet long can hold a pile of water, plus the inch or so in the tank.

Could u consider changing the height of water in the tank using your drains, I can adjust mine so water is touching the euro race or take it to water just above the overflow slits.

You can't be running that sump with the water level only 2" below the overflow? What's the water level height is that sump currently running at?

Isn't your refugium filled up all the way right now? Is it next to the return chamber? Is the return chamber normally filled only partway during normal operation? If these are all "yes", then you can switch up the baffling so the refugium is part of the return chamber and is only filled partway. You should be able to get more volume then.

I can't remember what your sump looks like, and no pic on first page or this page, so I gave up looking. :lol: