You'll need a little over a foot to fit and completely submerge a 125 watt Jager heater in a chamber that never empties. That sounds like a good amount of rock and sand. You can get live sand locally if it's cheaper than shipping from Tampa Bay and I'd recommend something not "sugar gain" sized but larger like special grade so it doesn't blow around too much.Lol...I just googled what is a mantis
Thank you. I understand now why a controller is so important and thank you for the recommendation. I'm trying to figure out size of heater I should fit back in the rear sump. Ok I guess now Tank( E170)+Rock (30lb- Tampa Bay)+Sand (40lb Tampa Bay)+ Temp controller ( Dual Ranko) planned.