120g tank with only one 1" drain in center overflow

[bluetang_2000]

Would this be a potential problem with only one drain?

[46FiatYamaha]

not really, you might not be able to get a huge amount of flow through the sump, but that usually causes more problems than it solves anyway. A few koralias, vortech, tunze etc... will help keep flow up inside the tank. I don't see anything wrong with that