AP nice looking pond! You can tell from the photo that you have great biodiversity. I'm a firm believer that the more types of critters you have in and around your pond, the better off your water quality will be. Nature truly has an amazing way of doing things right. The pH does seem to be a a bit low for bacteria. The thresholds for the bacteria we use is pH 6 - 8.5.
MM, if you are going to try bacteria, I recommend a testing program as well. Mark 3 or 4 locations througout the pond where you can test the sludge depth monthly. Start with the benchmark before treatment. In order to judge sludge depth we use a device that we call a "sludge judge" which consists of a long clear piece of 1.5" PVC tubing with a ball valve attached to the top. We draw 1" increments from the bottom up to about 4 feet. We take the sludge judge out to the testing plots, open the valve at the top and plunge it deep into the sludge until it stops. We then close the valve which holds the sludge in the clear tubing. Pull it out and see how far up the incrmenents the sludge accumulated. Test monthly to see how much bang your getting for your buck. For deeper water, you can cut the pvc so that it is mobile and then use a seal coupler to put the pipe back together. Also, clear PVC is expensive so you can use the coupling technique so that you only need a few feet of clear on the bottom which you can couple to the standard white pvc on top. Good luck.
