There are several things that could cause the temp in your tank to raise. It would help if we knew a little more about your setup.
1. What type of lights are you running?
2. How far above the tank do you have them?
3. Do you have glass covers over the tank?
4. How many powerheads do you have in your tank?
These could all make a big difference in how cool you can get your tank down.
Lighting - There are three types of lighting, Metal Halides - Very hot!!!, Compact Floresence- they are warm but cooler than MH, and T-5's - which are the coolest running.
If your light sit right on top of your tank they will blast the heat from the lights right into the tank. If you have the glass cover on top of the tank the glass just intensifies the heat. Just as the windows in you car.
Suggestion, Get the lights at least 6 to 8 inches off the top of the tank. Find a fan of some sort to blow air across the top of the tank. (Not into, just across the top) This will help to blow a lot of the heat from the lights away from the tank. Take the glass tops off the tank, replace it with eggcrate if you are worried about fish jumping from the tank. (Check out the other post on here, there was one not too long ago asking what eggcrate was. you can find a picture of it if you don't know what we are refering to when we say eggcrate.) This is very important in two ways, 1. the glass isnot there to intensify the heat from the lights. and 2. This will allow the tank to sweat. Proper evaporation in important to the tank just as it is for humans when it is hot. It allows for proper gas exchange. But keep extra RO/DI (Fresh, NOT Saltwater) water handy for toping off the tank.
The last thing is to check your powerheads. Remember there are little motors running 24/7 in those. And with that those motors do put off heat. And if one is not the best quality it could put off even more heat to heat up your tank.
Beleive it or not, I had a rio and an aqua med in my 180, the temp was running 87. and that was with the light off and at 7 am. After checking the powerheads and switching them out my tank hasn't gotten above 82.
Lastly if your home is warm that will help heat up your tank, Keep the AC on, or put a window unit in the room that your tank is in. and keep fans blowing cool air on and around the tank.
If none of these help, I am sorry to say you will more than likely need a chiller.