Cooling a tank is much more difficult and expensive as heating it. But you definitely want to keep it below 90. You can buy an aquarium chiller, or if you need to do it yourself buy a small used fridge, put the canister filter in there. You can then regulate the temperature by extending the tubes in the fridge.
Other options include putting a fan to blow over the surface, which increases evaporation and cools. But be sure to do more water changes, or top off with RO water. Look at your lights, as well--switching to LED if you have plants will help, or just turn them off if there aren't any plants to worry about.
I use water bottles frozen in the freezer, but they are only good for lowering the temp a couple degrees unless you use many and keep rotating them.
Ice melts pretty fast in a 90 degree tank.
The fan method also works farily well. I use both in conjunction during our hot summer spells.