I had a similar problem. My tank
was in direct sunlight for atleast an hour and a half of the day. It would often rise from 73 to 81 degrees in a matter
of half an hour! I didn't loose any fish because my zebra danios and white cloud minnows are pretty hardy however i took some precautions which lowered the
temperature considerably, keeping it nice and stable. First, i kept the lid off the tank constantly (my tank has a lip on the edge so my fish havn't attempted
to jump out yet- i know they can though), this allows the warm humid air that lies on the surface to escape providing cooler air to wash over the surface. I
also angled my filter outlet so water spurts out of the water, meaning that the water flying through the air has a chance to cool further. I didn't do this
next bit but i imagine it would be good to angel some kind of fan onto the surface of the water- this not only agitates the waters surface providing oxygen
(oxygen levels get low during hot weather) but it also blows cool air onto the waters surface, once again further cooling it. Now, these changes will reduce
the temperature by a couple of degrees (i find they get the tank to a stable 75/76 from an incredibly hot 81 degrees in my tank atleast). I agree with the ice
technique however, i placed a large sheet of ice around 2" wide by 10" long by 1" deep on the surface of my tank, i let it melt completly (don't be alarmed,
this is frozen tank water), the fish didn't mined it and even though it melted in a matter of about an hour it kept the water at a cool 72 degrees- perfect for
my coldwater danios- initially the temperature did drop to 69 degrees so i would remove it before it hit that stage. The reason the cold water stays cold for
the rest of the day is because water has a high specific heat capacity (it takes a large amount of water to heat up one kg of water by 1 degrees celsius)... i
can't remeber if it is grams or kilograms, correct me if i am wrong. Below is a link to pictures of my tank- you can see how my filter oulet works.
http
/s1069.photobucket.com/albums/u461/Clearwater_Pro/?albumview=slideshow
