Personally, I think a bottle of cold or iced water is a really bad idea. Many people cool their water this way, but the temp in the tank will differ a lot in different areas this way. Around the bottle (where a lot of fish will hide out because its cooler) the water will be too cool for them, while at the bottom of the tank the water is too warm. The temperature difference often causes disease and death.
I think your best bet will be, make sure they're no sunlight coming into the tank, try and keep the temperature around the tank as low as possible. Either by having AC on in the room if you've got it, or having a fan pointed at the tank constantly. Also, try to not leave your lights on too long, and perhaps turn them off at the hotest times of the day. The light bulbs tend to heat up the water a lot as well.
I've also heard of people wrapping foil around their tank to keep the temp inside about the same, but wouldnt this just warm it up?
Good luck getting the temp down
With just cories in a tank, it cant hurt to add slightly cooler (no more than 2C) water about twice a day. At the right temp, this can also cause cories to spawn, so
(Best luck I've had with cories ... I've got around 100 of them now
3 adults and loads of fry ... is at a temp of 23/24C)