Marie's 300g build

[marie]

ugghh, the mysteries of plumbing.



I have set up the plumbing in the overflow the way it was set up before.......could someone please explain to me the concept? I get the returns and the main overflow to the sump with a ball valve attached but why 2 emergency overflow pipes, both the same height.
There are 5 holes drilled into the overflow 2 smaller and 3 large is there a better way to do this? anybody want to come over and do it for me