Herbie sump ?

[mark]

I have one of my drains plumbed like that so some flow is bypassed to a dedicated refugium and it doesn't act like a Herbie.

If you closed the valve completely all you would be doing is just send all your water to the fuge. To have your drain behave as a Herbie you would need to restrict the overall flow from the drain so you would need to valve both legs. Could see it being a little difficult to set up.