I built a DIY Overflow to connect my 40G DT to my 20G sump and the 1" DIY Overflow is way too slow. In the video he's going at 400ish gph mine probably wasn't going even 100 gph. My return pump was outpacing it by miles.

I just can't figure out why mine is going so slow. I'll include a link to an image of mine for your thoughts. They way I'm priming it is by capping the bottom(out) and sucking on the open end air pipe at the top till i get water. Then I release the bottom. I feel like I'm having a very simple issue but I just can't figure it out.
A side note, in his videos he fills it with a bottle but that never works for me for some reason, I physically have to suck and pull water in the tubing from the tank.

Click here to enlarge