unplugging the heater and then adding cooler water will shock the fish. there shouldn't be more than a + or - 5F change within a short while or it will stress your fish. if the heater is faulty, turn down the nob to slightly decrease temperature until you can find a replacement. if the temps are too high, add some airstones or anything that will create a disturbance at the surface so that there is more dissolved oxygen in the water, which decrease as water gets warmer. easiest way is to decrease water level so if you have a HOB or cannister, it will waterfall down.