from what i know there can't be 2 different reading on the temp.. 1 of the reading must be wrong.. anyway if u off the chiller when the temp is at 28c, how are u going to maintain the temp or when are u going to on the chiller again?
i believe this depends on what chiller u are using and whats your tank size.. for me my tank is 4*2*2 with cl650 will run for around 20mins before resting for 2hrs plus