Problem with the pumps idea is that you'll never be able to match the gph. Even with idential pumps, the bottom one is going to have to deal with head loss. It would be impossible to balance practically in the long run. Your only options are to drill your tank or have a hang-on overflow, I'm afraid, unless you wanted to go with just a hang-on refugium type of setup.
On the upside, HOB overflow boxes don't have to be big and ugly, or complicated either. They are pretty reliable and you can always put the return pump on a float switch if you're worried about the overflow box failing. It's what I have done, and it's been working flawlessly for me for years.
|