any particular reason why you are leaning towards and external return pump vs internal. i prefer externals for a larger tank. you could even look at the Ecotech Vectra DC pump. that way you can crank it up for more flow through the tank when you need. you aren't putting in powerheads right?
as far as sumps go, i like to go with the largest sump i can fit in. more water volume you have the better off you'd be IMO. plus a larger sump gives you more room for upgrades/ease of maintenance
DC Pump is planned. VectraM1, Neptune Cor, or Jebao DCP were the ones on my radar. No powerheads in the display.
I totally understand the bigger the sump the better but I also need room for a top off container and small bits of equipment and media. I just thought external looked pretty cool.
