Aeration would be a good addition, since your pond is shallow a few linear air pumps would help, they are low cost and very reliable. Here's a brand that some of us use,I use the ecoplus 5, others use the larger ecoplus 7.
http://www.aquariumspecialty.com/air-pum...ommercial-air-5
You could also buy a much larger pump and run long lines to the diffusers.

Bacteria can also be added, some people use it in conjunction with aeration.