Ok for those who aren't made of money and can't just up and buy a nice chiller to save your tanks from becoming boiling pots. Here are a few remedies for your average fishkeeper who isn't "too proud" ..
1. Cut your lighting cycle to only 4-6 hours and if you're going to be gone at work/school all day, make that cycle at night, so during the hottest part of the day, you're lights are off.
2. Add a fan(s) blowing in the direction of your lights to remove the additional heat they give off, and whisk off surface heat from the water.
3. Get some gallon sized zip-lock bags, fill them 2/3 with tap water, freeze them, and then add them to the tank. They will float, but you can secure them if you don't want them floating all over. Best place to put them, is infront, or near the return flow, so it's cooled as it's returned to the tank. I'd suggest only adding one ice-bag for every 40+ gallons, as you don't want the temp cooling too much, or too quickly. The tap water will melt in the bag - not in your tank, and you can just lift it out and re-freeze, and swap for another that's already frozen if you need to.
It's easy, it's effective and it's cheap.
Keep those temps between 73-78.