Should I use this?

[galantra]

So I have a 150 cube but have a third hole drilled and within my corner overflow there is a lower portion cut to add in a close loop. But I want a clean open top so now I am no longer doing a close loop. Wanted to hear some options of use or should I just plug it closer?

Have a look

 

[Hitman]

Personally I would plug the hole into the overflow ( the black plastic ) and then use the 3rd hole as a emergency overflow into your sump to prevent your display from ever overflowing.

 

[W1ngz]

That's a terrifying place for a drain. The slightest glitch in plumbing or a pump failure, and it's going to let all 150 gallons of water free flow right out of the tank.