Re: 20g High Vivarium w/ Pond Build
I think a stream going from one corner to the other would be attractive, if I'm understanding correctly that the pump is in that little cube-shaped recess and the pond is in the other recess in the other corner. I'm afraid I don't know enough about insect life to know if they'd be endangered by a slow-moving stream in any way. I wouldn't think so.
My general experience with waterfalls, though, is that the shorter distance the length of pipe goes and the shorter distance the stream goes, the less likely things are to fail to do what they're supposed to do. My alma mater had a somewhat elaborate ceiling-to-floor wall waterfall that, by the time I got there, wasn't really covering with water the whole area it was supposed to cover. Before I left it had been shut off and the pool at the bottom converted into a plant bed.
I don't know what kind of stream you're picturing, of course. If it's running through a simple groove in the ground, it's a lot harder for it to screw up than if it's supposed to cascade gently over several different surfaces. In the latter case, some slight upset could make the water just decide not to spill over part of it.