I have a very similar problem, with plants curling and drooping without reason... I've around 20 babies all in a flood n drain style system with lots of airstones, all have the same Canna nutes but every one has its own strange tendencies. I've found it's not the Ph as I keep it between 5.4 - 6.2, overwatering will definetly droop the plants but i've found oxygen to the biggest factor in leef curling. I had a few plants looking almost dead with no spring in the leaves perk up and reach for the sky after sticking an air stone under the pot. My plants now are great, the only time they droop and curl is when the light goes off, and when the timer kicks in they go from drooping down to raising up in around 10 minutes... The best the plants were ever doing was when they were sitting in water being bubbled 20 - 22 hrs per day they were going great and even through their dark period they would stay upright and straight. Using the same system and technique on the same plants, they are now doing the droop thing again whenever the lights are off. Im putting it down to a being winter thing Snoop's on the money, with those pics showing Ph and Overwatering probs. nice one