Welcome to the forums HaBI!
I can't give any direct advice as I have not had to fight a watermeal/duckweed explosion, but I can certainly direct you to the area of the forums where you can find some good reading on the subject...
http://forums.pondboss.com/ubbthreads.php?ubb=postlist&Board=14&page=1Scan the thread titles for Duckweed or Watermeal for some conversations on the subjects.
The best way to search the Pondboss forums is to go to google search and type in...
site:forums.pondboss.com Duckweed watermeal
This will search the forums for any threads with the words duckweed and watermeal. This works much better than the forum search function and the above search yielded at least a couple pages of threads talking about the control of those plants.
A can say that an aeration system is not going to solve your troubles. It may help, but not much I'd bet.
