I also think its probably due to excess nitrogen, thus all the bubbles. I had my water tested and was told i could benefit from lime and fertilizing. This first round of fertilizing was really just a trial and error run since I haven't stocked yet. Would you pull out the bag that has the remainder of fertilizer from the pond?
Lime alone often raises pH enough to allow the uptake of dissolved nutrient already in your pond, without additional fertilization.
The bubbles are likely trapped oxygen produced and or carbon dioxide respired from the BG algae bloom
