My koi and goldfish are dying in my pond and we are almost positive that it is from parasites in our pond. Does anyone have any idea as to how we could get rid of the parasite completely? I don't know if that's a thing that you can do by fully getting rid of them but I'm almost positive that is what is killing my fish. My fish I found dead this morning was completely intact and when I took it out of the pond it was bleeding from the other side and I couldn't see any holes or anything like that. I want my fish to have a good environment and be able to thrive. We clean our pond out once a year in the spring. We remove the fish and we drain the pond and then pressure wash it and clean it with soap and water and pressure wash it again to make sure it's fully clean and we fill it back up and it stays clean for a few months and then goes back to the way it is now which is full of algae which has turned our pond water into a dark green color. I'm fairly new at taking care of ponds but I'm definitely wanting to learn as much as I can. We have a fairly big pond that is bigger than a normal backyard pond. If anyone has any advice, it would be much appreciated.

If your fish seem to be bleeding, the only thing I can think of is a disease called Hemorrhagic Septicemia, which is basically internal bleeding. The root cause is poor water conditions, such as high ammonia, nitrates and nitrites. The first thing I'd do is test your water. Ammonia and nitrites should be at 0 or very close, and nitrates should be lower than 20ppm.

Other than that, I'm not sure.

Sounds like Koi herpesvirus (KHV). Not sure how to eradicate the virus in a closed system but it's worth looking into.