I have a 75 gallon pond with a high ammonia level of 4.0. I had three large goldfish about 8-9 inches long in the pond with some plants. Noticed they were on the bottom of the pond and took a chemical reading and found the ammonia level was way up. Removed the fish indoors to a 55 gal tank but this can only be temporary as there is not enough space. Should I remove most of the water and start over? What caused the high level? too many fish? the plants? I Have a UV filter outside and a sponge filter underwater. PH was 7.6 and nitrites were ok also. Nitrate level was 5.0. That could be better. Sounds like pollution problem but not sure. Any advice out there?