Good news!

I contacted Marineland last week to see if my 27 gallon cube could be drilled and it can. So now the question is, where should i drill the holes? The dimmensions are 20"x18"x20.

 

well where do you plan to put the overflow? the hole is usually drilled at the base of the overflow

 

I was thinking on the upper left hand corner, but i dont know how far in to go or anything like that.

 

This all depends on how you are going to plumb the tank. Check this thread out, it shows a 20 long drilled with options for 2 overflows and 1 return or 1 overflow and 2 returns.

Drilled 20L and stand for sale - Reef Central Online Community

 

Thanks, I will take a look when i get home, im at work and pictures dont show up.