28 degrees is nothing. My tanks reached 34+ last summer. There was no avoiding it. Keep the lights off, make sure the windows are open to let in a cool breeze (if there is one!), take the lids off and direct a fan over the top of the water. Ice filled bottles are fine to use IMO
. If there is good water flow in the tank there won't be cold and hot spots. Only problem is if the temp drops too quickly.