it's a pretty small tank, try taking the cover off the HOB filter, you will have water evaporate, but that is what you want. If you are still having issues, leave the tank uncovered, or a screen top. If that all doesn't get you down to 86F/30C or below, get a fan to blow on top of the water. The air movement ought to do the trick. Low tech options are usually better options than a higher tech options. Also turn the light off, and make sure the tank doesn't get to much sunlight.