You don't have to stress to much if the temp only reaches around 82 degrees, this is still rather fair for a betta to be in. You should only worry if its getting to 89+ as this may stress the fish, or if the temp worsens it may be fatal. Some things you can try, and I'm uncertain if this will wok effectively; Placing fans in the room its in (Industrial fans, 1 foot in radius, is what I have, works pritty well to decrease the tmep int he room a few degrees), keep the tank in shaded areas, decrease the amount of time your tank light is on on the hood.
To give you a reference point, my house is incredibly shotty and I have a poor air conditioner as well, so the house temp usually sits around 88 degrees when its really hot outside. The tempreture of the tank stays rather close behind this, and on a hot day sits around a constant 85 or so for the daylight hours.
The heater in my tank is a tetra product that doesn't activate unless it reads the tempreture of the water hitting 77 or less. So I don't worry about this adding to the water temp on hot days. The only thing you might have to worry about if your heater isn't working at all, is the sudden decrease in temp during night. Where your fish has adjusted to the 82 degrees right now, at night when the water shifts to possibly house temp of 72 (If your heater isn't working in the tank) then this can place alot of stress on the beta.
Last edited by Kokonoko; 08-17-2010 at 05:59 PM.