All depends on your temp, lights, pumps, etc. On my 79G tank and 10-15G sump with 2-150w mh and 4 39w T5's the chiller would turn on several times a day for less than an 1/2 hour at a time. Once I upgraded to 2-250w mh and 4-54w T5's it started running over an hour so I upgraded to a 1/5 hp. !50G may be a bit large for this chiller, what type of lighting do you have?