I have this problem in my Via aqua 620 tank, i found ice packs no good. like wilder says the effects are short lived + i was always worried they would squash a fish. Now i leave the lid open as much as possible, leave the lights off while the temps up & turn on my ceiling fan on this all works a treat for me
drape a damp towel and a fan over the tank (put the fan on a timer if you not home so it comes on mid morning if needed) can lower the tank temp by 2-3C depending on the size of the tank.