Thanks for bringing that up. Unfortunatly, I dont answer at most of your questions. My plants were fine, even , they looked more healthy when I got back than when I left! They were bigger and got nicer colors. The temperature were a little colder in one tank. But I dOn't think tha tit happened suddenly. I left for a week and after reading few posts here, I decided to make them feast during that time. So the last time they ate was the morning I left and I gave them the same numbers of pellet as usual. The night before, I did a 50% water change since I did a 100% change 5 days before. Finally, I let the light of the aauqrium on all the time.
In my opinion, I think that the water evaporation killed them. The water level got too low and I guess the filter succion began to be too strong and got them stuck. It's my opinion...I had the lid on for one of the tank (the one made by Hagen for the Chi) but the evaporation was almost the same in both tank. Maybe 2 cm difference.
My third fish in is Fluval Spec was perfectly fine and is quite happy to be feed daily now! lol
Anyway thanks for your input, I will be vigilent in the future about the factors your brought up. But let me say, that I,m nervous to let the filter runs when I'm away now...