Soya,
just like i said...the air-con must blow to the tank directly then only u can see the result. OR...you hv a very small room and the aquarium is inside the room. (but then you or your family members cant enjoy the air-con since u just implement inside the room for aquarium.)

if implement it in living room...you still can't get that kind of result. Temp will be still maintain at 27-29C. I believe soya not yet try....once you tried. you will knw. Like you said : if got light as long as you don't have too many electric gadget inside warming up the water. I think the lighting enough to increase the temp of water.