How I can keep my tank cool in the summer?

I have a 30 gallon tank with bio filter wheel nominal rate of 50 g. I live on the fourth floor of my apartment building and the aquarium sits on the floor. I take care of a large orange Orada and a Black Moor. I reduce my electricity consumption, so I'm trying not to run the air conditioner as much as possible. The problem is that when outside temperatures are in the 80 and 90 (I live in Minneapolis, MN), temperature Room. although smaller than the outside temperature is still high. I have 2 of the adhesive type thermometers in the tank and saw the temperature range of 70-84 degrees. I the tank would be in the low to mid 70's. Any ideas on how I can keep the temperature on summer days?? I follow the light hood all day and now I have a small fan blowing across the surface of the water cool by evaporation. My other idea is just to get some of reusable ice packs, put them in a plastic bag and float 1 or 2 in the water. Thanks !!!!!!

You can try using ice-Ziplocks cool the tank, but that usually results in a temperature roller coaster that can be worse in the fish that the temperature is too high. 84F is not too hot to fish for a short period, as usually. Unless you are seeing signs of significant distress, I would say not to worry and just let the tank temperature to move naturally. I realize the mid-70 is often given as the preferred range of temperautre for fish, but in the mid 80's do them hurt either. MM

Nano Lagoon (7): Thermometer