I agree...soda bottles or plastic bags can work. Also, if it's time to add freshwater anyway, then (RO-water) ice can work. But still...if you work out what's involved, ice just isn't a practical solution for daily use...it's really only best for the odd situation where something has gone wrong and the tempereature has spiked a lot. If you can't work something out with making sure your lights aren't too bright/too close, using room AC, having an open lid or sump, and a fan actively encouraging evaporative cooling, then you need to consider a chiller.
rusty