Sump pump size...

[mark]

From first hand experience have to disagree that a modified Herbie is a problem waiting to happen. Mine's been up since last October, water in the 2nd chamber can't be that bad as snails don't seem to find it a problem. As for sediment, what's in the overflow is what's left from when I pulled out the Durso. If it was a problem guess I could just siphon out when I did a water change.

Idea for the emergency being a Durso is interesting but since I'll need to worry about flooding at 2000gph if the primary became blocked, I'd be reluctant having the emergency standpipe anything other than a open pipe.

Drain size, here's a calculator for a guide, (towards bottom). 1.5" should be able to handle a Dart.