drop res. temps - dry ice??

I have a grow tent and I have to keep my complete system in it. I have a DWC setup with a 20 gallon res. and 4 buckets with a drip system to them. All bodies of water have air stones coming from a cold air source (open window, the pumps are on the window seal). Light is 1000 watts, running at night only to keep temps down but my res is getting up to like 79-85 F. I have put frozen bottles (2 liters) in there and bags of ice but it all melts within an hour.. even the 2 liters!! A water chiller is not in the budget just yet since i just bought this whole new system (if you look at my last grow it was a 400 watt system) BUT I am bidding on one as I type here. If I do not win it, is dry ice an option? Can I just put a chunk of dry ice in my res? 1 lb of dry ice I would assume would keep it cool all night for the next few days until I work a chiller in to budget. Thoughts?

Ever put dry ice in warm water... Lmao it's funny. You get a fog show, and the dry-ice doesn't last very long.

Yeah I have done it for Halloween. What if I get the water down to 60 before doing it, is dry ice an okay idea for a short time OR does anyone have any helpful ideas on this? The bags of ice and 2 liters got it down to 60 tonight but its all melted and back at 70 now.