We have a 50g with an external overflow as our tank is not drilled and a 13g sump underneath. We still have some surface scum if one of the powerheads has slipped and is not agitating the surface and moving water towards the overflow...we also have the prefilter box on our Remora Pro....yup they take up a ton of space...we were at least able to get the heater out of site...but nothing else. Both of our other tanks are drilled. To do it again we would
only go with a drilled tank..they are the way to go but a big pain if you have a set up and want to change it..unless of course you go BIGGER! and just move stuff over!
Our water doesn't move overly fast through our sump but if it did we worry about micro bubbles with the sump being so narrow...good luck...you can do it without a sump etc but....