Raul,

Sorry about your 50 rainbows!

Yes, there have been discussions about using supplemental cold water and even some discussions of geothermal heating/cooling loops. (Hit the search icon, and then play around using the Advanced Forum Search function.)

Is your cold tap water chlorinated? How much dissolved oxygen is in your tap water? If you need to flush chlorination and add oxygen, there are some discussions of how people treat low-oxygen well water before supplementing their pond water.

I suspect in your case (and Mary's case) that if it was possible to get a supply of cool, oxygenated water to the deep section of your pond for even ONE WEEK, then a lot of trout would have survived that brutal heat wave.