To be super safe for my 120 display + 93 gal cube all plumbed to my 40 sump i have 2 heaters in the sump and a heater in each tank. Just in case one or two go haywire i have backups. All controlled through the apex.
I orginally had 2 heaters just in my sump until that one special day when both died so thats why im over cautious.