high ammonia level

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?

 

I changed out a good amount of water and added Amquel for ponds when mine got that high. I actually don't know what happened to mine but it came about just fine and the fish continued to thrive. I had a big, fat Jack Dempsey in there with a couple koi.

 

Discovered that the high ammonia level may be related to the germination of the cattail plant seeds in the pond. I noticed that hundreds of seeds were spread throughout the pond. Drained it and moved the goldfish to inside tank. Cleaned out seeds and the clogged filter and replaced water and let it filter 4 days. Checked chemical levels and replaced fish when levels were normal. Observing fish which appear to be ok so far.