I just found this forum and appears it can really help us novice pond owners! I built a new pond back in July and in August started to put in koi fish. Now the fish are all starting to die! My pond size is 6' x 12' and is 4' deep, about 2100 gallons. I added filter material around the submersible pump (I use the output for my water fall feature) however, the filter is not doing the job. I'm wondering if I need to add a second submersible pump and connect to an external mechanical filter for filtering out the pond. I'm frustrated because the water is very cloudy and full of algea. Not sure what to do here but I hate seeing fish die.