There are several small chillers, commonly called micro-chillers, that can be used on nano aquariums to drop the temperatures a few degrees. They often require you to drill a hole in your aquarium, as they mount somewhat like a bulkhead fitting. However, if you don’t want to drill your aquarium, you can mount the chiller to a pump or filter. Simply drill a hole in the top of hang-on filter or the side of a canister filter and mount the chiller. Make sure the chiller’s probe is submerged in the water or it will not be effective.