Yes droopy leaves can be a sign of thirst, but that's usually not the problem in hydro. Just the opposite is more common in hydro. Not enough oxygen to the roots and/or too high of water level on the roots is much more common, and can also make plants droopy. You need to get as little water as possible to the roots, believe it or not.

However, there may not be a problem at all. My plants droop, but not at night. They wake up nice and refreshed, standing straight and tall without a droop among them. Then as the day progresses, they seem to get tired after their long hard day of trying to suck up nutrients, etc. Try looking at your plants before the lights come on. Are they drooping after 6 hours of rest? If not, everything is probably fine.

I would not put dry ice directly in a reservoir that roots are in. I should think it would give the roots frostbite, or at least shock the plants in some way. Also lower temps make the plant grow slower. The absolute best thing you could do for your plant is to get a bigger air pump and triple the amount of bubbles you've got going at the present, and leave the air running 24 hrs. It would also be beneficial to get a power head, which will circulate your reservoir.