As far as drilling goes, the sump (the one on the left) is acrylic and can be drilled, but the refugium (right) is glass and can't be drilled. At one point I had a giant U-tube connecting the two but I switched it to the overflow design I have now because I wanted to get more water volume by filling the sump as full of water as I could.
Lowering the water level in the sump is certainly a possibility, but I'm afraid that if I lower it enough to catch all of the excess water in the system that I'll lose that 10 gallons I was trying to save by going with the overflow design in the first place, which would mean the water level in the sump wouldn't be high enough to use that overflow at all. That would basically put me back to where I was before with the giant ugly U-tube. Come to think of it, I have a picture of what that setup used to look like:
The "waterfall" idea I mentioned before might be a solution that only involves drilling the sump. I threw a drawing together in MS Paint and I'll try to explain what my idea is -- hopefully...
The black tank on the left is the sump, the blue on the right is the refugium. The green tube represents the overflow that's there right now. That keeps the water level below a certain point. The part in red is what I was thinking of adding. If the overflow ever fails and the water level gets higher, it would get to that part I drill out of the sump (maybe 1/2" lower than the top) first and just spill over into the refugium. I suppose the same thing could be accomplished with drilling and PVC
Does this design look feasible?