When Nemo first tried to clog the filter (with a rock) he had to go through the tube to go in and out. But when he did it the 2nd time, the tank gang had a plant going up the tube, then Nemo appeared. How did he get out if there was a plant in the tube?
Excellent, I spotted that too. But maybe he could've jumped back out through the top of the aquarium where that turning thingy is, like how he went in...who knows?
First of all, he did not go up the tube to get in the first time. He went over the top propeller. You'll notice that going over that was easier than going through the tube. So, when he tried to come out the second time, he just went through the easier way, over the propeller.