Whoa how could anyone even attempt to answer this without better information?

From what I could tell the chiller is rated for about 65g as being the most efficient. You don't list the size of your tank, how much water you are pushing through the chiller (to quick and you won't have a heat exchange at all), what your temp was before and after, where did you locate the chiller? Way to many variables to give an honest answer. See if you run water through the chiller at more than 400lph (about 80gph) the water will not have enough time in the heat exchanger, it will just shoot past the work area and back into the tank. If the chiller is in a sealed cabinet or closet, or to close to a wall it can prevent cool air from blowing over the exchanger. If your tank is 120g and you have a lot of equipment running.... Blah blah blah.....