The main issue with drilling holes is the lack of flexibility, more often than not you need to move returns around to match the scape.
I use sumps on most of my tanks & tend to cut out an overflow slot rather than drill a hole!
There is one safe return configuration that always works.. it is a spray bar, so I often drill a single 25mm hole in the middle back pane, about 30mm down (centre) & use a T to feed a spray bar. However I cant seem to get clear 25mm Ts anymore so the last one I drilled in the side.
Of course marine tanks often have mutable returns & twin overflows but I still prefer to cut a slot & keep return holes away fro the bottom!
Drilling a hole is a commitment, unfortunately the hole is rarely in the right place once you get to know your tank.
I use sumps on most of my tanks & tend to cut out an overflow slot rather than drill a hole!
There is one safe return configuration that always works.. it is a spray bar, so I often drill a single 25mm hole in the middle back pane, about 30mm down (centre) & use a T to feed a spray bar. However I cant seem to get clear 25mm Ts anymore so the last one I drilled in the side.
Of course marine tanks often have mutable returns & twin overflows but I still prefer to cut a slot & keep return holes away fro the bottom!
Drilling a hole is a commitment, unfortunately the hole is rarely in the right place once you get to know your tank.