Burgermeister is correct, aeration isn't going to solve your problems immediately, it may take some time to get the pond back into balance. It sounds like you have a properly sized system for your size pond, it isn't under or over-aerating. The gas being released upon start-up is normal, since your pond was anaerobic to start-out with, the shift came when you got rid of the duckweed and then the sunlight could penetrate the surface and cause filamentous algae growth and planktonic algae growth. Pond dye should help with the filamentous and planktonic algae is actually a much more desirable type of algae than many other types since it is the basis of the food chain. I wouldn't worry about the diffusers disturbing the bottom sediments, they are designed not to do so ~ and only in very rare occasions is the bottom sediment so loose and unconsolidated that it gets entrained into the bubble column. As for the bacteria you are adding... the jury is still out on whether or not it works ~ I would just give your pond a little time to work through its inbalance. Have you consulted a pond manager to take a look? Maybe adding tilapia would help control your algae in the mean time?
