please help me understand plumbing

[Reef-Geek]

Hi, I bought the aqueon plumbing kit and the return pipe makes random air bubbles, I think I can solve the problem in 2 ways
1. turn the return valve half way, I am using eheim 1262 which maybe too strong for the 72 gal but turn it half way is just reduce too much flow
2. like the picture shows, there is a hole as marked, if I use finger to cover the hole the bubble's gone, what is that hole for? is it needed?

so what can I do to reduce the bubbles? thanks
ps. the I don't think they are microbubles.