Check the temperature in the tank right before the photoperiod ends. I had this happen to me in the past, and the temp in the tank was going over 80F, due to the heat generated by my lighting. I had to adjust my wrap so cool air can go in on both ends, and move my lighting up.
Unfortunately, at this stage, I think it might be too late to keep it emmersed. I would echo what others have mentioned by flooding the tank and cranking up the CO2. If you don't want to do that yet, then you can check the temps, make adjustments, but be prepared to add more HC if more dies before they bounce back.