I'm thinking 3" since I'm not really doing a settling chamber, just a pre-filter.
I'm just not sure which one I like best. To be honest I have the left side already existing 16'x5.5, If I wanted to really save $ I'd just build up to it with the new side and "act" as if they were one pond, the bridge will be over that area anyhow. I could seperate the fish that way, 'blues" on one side

But I really need to put an inliner BD there, and I wanted to dig it deeper and wider,
I know if I just make the new side, I'll regret it? I thought about seaming it, which I could do with the shallow area in the middle. BUT it scares me, seaming. I'm kinda leaning towards the Deeper in the center, the fish would hang under the cover of the bridge that way. Oh what to do, what to do:banghead3:

:banghead3:
What would you do?
Things I'm planning on buying, BD's(1 or 2), Submersible pump, edpm liner 20x40. Misc items are inerds for the skimmer, piping etc.