Try a PC
/USB cooling fan to disperse heat from the surface of the water. It won't give you drastic temperature change, but it'll bring it down a few degrees. Especially if you have heat coming from your light.
The downside to this is that you need an open top tank, but if you're having heat issues, that's a better idea anyway.
You can also switch the light schedule to opposite of actual daylight hours. Plants don't have a watch. They only care how much light they're getting, not when. So you can have the light on during the cooler hours and off during the heat of the day. You can also switch to LED
lighting, if you haven't already, since it runs cooler.
If you are using an external, canister filter, it will cool things down anyway since the water will have more surface area through the tubing, cooling it to room temperature faster.
What are your current and target temps?