Heater wattage for 2 heaters in sump

[Quagmire]

It's a good idea to go with 2 heaters.What I have is 2 heaters, each rated for 1/2 the tank volume.A 120g tank would be 2 heaters rated for about 60g each.This way you won't over heat if 1 gets stuck on,and you won't cool off right away if 1 quits.But always check water temp.as part of your daily matenance.Good luck