pvc overflow

I've just finished building a PVC overflow system for my 400 gallon tank (tempered glass, can't be drilled) but I've run into an issue where if I just have the valve fully opened, it empties the internal weir (90mm pvc pipe with one end capped and a mesh vent over as the weir 'teeth') which...

I have a PVC overflow on my pool pond. The output of this overflow is only about 3-4 inches lower than the water surface of the pond. It doesn’t seem to be draining the correct amount of water per hour from what I’ve read online. I’m using 2” pvc. My question is does the height of the output of...