I am no expert, but my ponds went green when first filling, I believe because there were a lot of nutrients in the soil within the ponds that had not had a chance to be used by pond plants and algae yet. You may still have survivors in the pond. It seems like stocking FHM is often recommended in your situation, but I am not sure that GSH are as tolerant of low DO. I personally have waited until full pool before stocking, but I seem to be in less of a rush than most. You want to be sure to stock what you want before something less desirable stocks itself though.