Yesterday, I purchased my first aquarium from PetSmart. It is a top fin 20 gallon tank that came in a starter kit with a filter, chemical conditioning, ect. I set the tank up easily, conditioned the water, added the gravel and decorations and put in the thermometer that came with it. I ran the tank through the night with the light off. Now the thermometer which is a thin plastic one that lights up numbers in multiples of 2 depending on the temperature. When I checked it the numbers 80,82,84 where all lit. From what I’ve read, for freshwater tanks it should be anywhere from 72-80F. The tank is near a window, which is always closed a no light gets through, a ceiling fan that runs fairly fast, and an air conditioning vent. I live in Texas where it gets very hot during the summertime like this, but I guess I just need to know how to cool it down and keep it cool. By the way, when I do get fish I plan on getting a rubber lip pleco, a ghost shrimp, a mystery snail and maybe one other fish. Also should I probably go buy a new thermometer because this one is kind of a cheapie. Thanks in advance 
Oh and I’m not using a heater because I was told not to by the person at PetSmart because of the already high temperature.
Okay thanks for all the answers, I got a new thermometer and it turns out the temp is at 79, so the old thermometer must have just been crappy
