Welcome to TONMO, the premier cephalopod interest community. Founded in 2000, we have built a large community of experts, hobbyists and enthusiasts, some of whom come together when we host our biennial conference. To join in on the fun, sign up - it's free! You can also become a Supporter for just $50/year to remove all ads and enjoy other perks, like Site Search. Follow us on Twitter or Facebook for more cephy goodness.
I have read of freezing water in small plastic coke bottles, keeping a supply of these in the freezer, and putting one in the sump whenever necessary. But I've never tried it - might be too extreme a temperature change.
I had a block of 4 CPU fans on a coral tank - they were directed at the water surface (parallel to the surface, and facing down) and they did quite a good job of cooling. They don't last long 1in above saltwater, though.
Now I have a cheapo 6in clip-on fan blowing directly into my sump, and no more coral growing lights. THe temp has gone down to 72 degrees! The only caution there is having any metal rust fall into the water.
I've tried the ice method, but in my opinion, it's too harsh and labor intensive. But it works in an emergency.