The laws of physics sya that two objects in contact with each other will reach the same temp. This means if you have a resrvoir in a 80 degree room inveritably the reservoir water temp is going to end up at 80 degrees. There are ways around this. A few have been mentioned but only paratially. The most impoirtant aspect in regard to cooling without a chiller is evaporative cooling. ie a fan drawing air across the waters surface. Drawing not blowing. More efficient says the physicist. Alos mentioned was that at most the temperatut re drop willbe to 5 degreesbelow the roon temp where the reseroir is located. Not entioned is colling the water out side the room where the reservoir is stored. Doing such will allow for colling to at least five degrees below the air temperature where the cooling is taking place. There are several ways to easily accomplish this with the choices being the stealth level needed. Consider if the stealth need is minimal that ypu can have a small reservoir just hust higher than your present reservoir placed out side your grow area. Pump water up and into the second reservoir and place a uniseal in the side with an overflow pie to allow excess water to flow back to the regular reservoir. Place a fan over this second reservoir drawing air across the waters surface This actually works best if it is done with a reservoir with a lid. Cut a slot or a series of holes just above the high water mark set by the drain you placed in the side of the reservoir. This hole fr or the drains should be near the top of the reservoir so that only a few inches of air space is between the surface of the water and the lid. A simple muffin fan or two is adequate for air movement. This will keep your reservoir water tempa t aprroximattely 5 degrees below the out side rooms air temperature. If you need additional stealth then use an aquariy um for the second reservoir woth a fan over the top of it to coll the aquarium water. Then pump water with a small pump in a circulatinig loopetween the aquarium and the reservoir. Instaed of pumping water into the reservoir buy a piece of titanium tubing to be placed in the reservoir. Push the tubing tightly over each end of the tubing so that aquarium water flows through the vinyl tubing then the titanium tubing and back through vinyl tubing to the aquarium. Oh yeah throw a gold fish or something in the aquarium.
If you really serious about growing you can put together a much better large chiller (1 hp) or about a fourth the cost of buying a comparable one or half the cost of a cheap one of that size. And the compressor would be outside instead of inside. A 1 hp chiller will easily handle the water chilling need for over 5000 watts of lighting.
How much the chiller can lower the temperature is entirely dependant on the size of the chiller and the wattage of the lights and where the reservoir is located. Obviously when possible depending on the system the reservoir should lawys be in a cool place. That usually means outside the grow area when/where possible. Otherwise the reasrvoir should be insulated if chilled to a tempearture bek low the average air temperature in the grow area. Typically a reservoirs water should be 10 to 20 degrees below the grow room air temperature as most grow rooms have air temps of about 75 to 85 degrees.
Figure heat gain to be removed as such. with a 1000 watt light 85% of the output is in heat producing wave lengths. With water cooled lights 75% of that heat will be extracted by the water flow through the tubes. There are 3.41 btu per watt. (1000 X 0.87 x 0.75 x 3.42) = 2174 btu A tyipical 1 hp airconditioner is rated at 12000 btu. (12000 / 2174) = 5.52 (1000 watt lights)
Without the water cooled light all the heat output of the light will be converted to btu of heat energy. What is not removed through ventilation will be taken up by the reservoir of water and surrounding obvjects or loss through convection etc through the walls, ceiling and floor. essentially the 0.75 will be 1.0 so (1000 x 0.85 x 3.41) = 2898 btu or 1/4 hp per each 1000 watts of light as a 1/4 hpair conditioner chiller has a capacity of 3000 btu.
Actually a chiller is more efficient than an air conditioner as water is a better thermal conductor than air. But no reason to get that specific.
Before you start wasting your time considering using a dormitary refrigerator as a chiller first consider they usually are only about 1/12 hp. That means they have a btu capacity of only 1000 btu. That means it can handle a light heat output in watts, if running constatlty, of only (1000 btu / 3.42 btu/watt) = 293 watts. As a HID's light puts out 85% of its wattage as heat producing wavelengths of light then a dormitory frig can handle the heat output of a (249/0.85) = 293 watt light. So with additional room ventilation and convective heat losses it can probably handle cooling the reservoir in a room lit by a 400 watt lamp.
As for reef tanks. They use more wattage then my small grow rooms. Between pumps and lights they cook without chilling. I make chillers out of room airconditioners with nearly all parst coming from eBay. Even good titanium heat exchangers can be bought there. I use them all year round even winter and it gets very cold where I am (-35 to -50 degrees F). The chillers compressor is outdoors but the heat exchanger is indoors.