I don't remember being asked by "customs" whether or not we are bringing live plants. They'll x-ray your luggage for pork and alcohol, but I don't really think that they'll care about the aquatic plants. You might want to check with a local aquarium store to see what they have to say.

If you are able to bring the plants in your carryon then I would try to do so. Just wrap them in damp newspaper and then put them in plastic bags - you'll risk overheating or very cool temps if you put them in your luggage.

I'm so, so jealous of your aquarium. . .wish I still had time to pursue that hobby!