Obviously, you can add a chiller to the aquarium.
However, you can also lower the water temperature by cooling off your equipment. The simplest way is to mount a very small fan to blow across the light stips. These fans can be purchased at any computer store, as they are intended for home computer systems.
If you have a canopy on the aquarium, is it well ventilated? Canopies need to have slits in the top to allow for heat to escape.
Have you checked the obvious, meaning your heater? Perhaps it is stuck in the on position.
This is off topic, but i am concerned about the long term success of your aquarium. Seargent Majors are not a good selection for an aquarium this small. They get large and are rather belligerent, requiring a much larger system for permanent success.