Robbie welcome to the PB Forum. I think we can help. We need some facts. Were they BG or HBG ?

Do you know for sure that you had a DO event that killed fish?

My gut reaction is , if you did not see a massive fish kill , then its not the water but the fish. Most times even a DO induced fish kill will not kill all the fish.

If you stocked 500 HBG and caught a 1000 + you were out of BG . They don't reproduce much. The CC and LMB could have lived off of pellets and eating their own for a while but with no recruitment the survivors died of old age. Natural morts can be high after 10 years.

Do you have access to a fishfinder and boat? If so go over the pond and see what it shows. Fish traps and seines will also help see what is there. If you don't catch a fish over a period of time ( say 3 hrs of total fishing over a mth.) using live bait and the other methods don't turn up any fish you will know it is time to start over. My first thought is to add some adult BG (male and female) now so they can spawn.