Read through this Aeration Thread from our Achives section. Near the bottom I put numerous links with good information about numerous topics about homemade or do-it- yourself aeration. Do lots of homework before pulling the trigger. A poorly built system with too little mixing is IMO a waste of money both in parts and with a system that wastes too much electricity and does not result in efficient mixing of the pond. Inadequate aeration is discussed in a couple of the above links. IMO it better to have too much aeration than too little. With overkill aeration you can do less than 24/7 operation each day. Reduced pump run time lengthens the life span of the compressor the most expensive part of the system.
The basic rule for aeration is the more bubbles you make and the smaller those bubbles are the more water you move. This information came from Aquatic EcoSystem (Pentair) hints in their catalog.
Too small of bubbles tend to clog more quickly that causes extra maintenance.
From your picture it looks like you may need to have 2 separate diffusers to get good top to bottom circulation throughout the pond.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92494#Post92494
