The only reason why I had to get a chiller was because when my in sump protein skimmer runs the submerged mag drive pump runs so hot it heats the water up to 87 degrees.
Since I only run my skimmer at night, I was having huge temp shifts. Luckily I noticed this while my tank was cycling, so only lr and ls were exposed to these huge temp shifts.
The chiller has a temp regulator like a refridgerator. It comes on when the temp starts climbing (usually 0.5 degree shift). So, when my lights heat up the water during the day, the chiller kicks on to compensate.