Can't get my aquarium out ammonia cycle

[Chrisinla]

I've had a 50 gallon aquarium with 4 goldfish in it for a few years now. I've never had a problem. Out of the blue the tank started showing a lot of ammonia and nothing else. I took the plants out bc they were slowly dying and adding to the cycle and added some media to my two filters (aquaclear 70 and a fluval canister). I also added an undergravel filter. It seems like it has been about a month now. What can I do?

 

[libertybelle]

How big are the goldfish, and what are the other parameters. (Particularly nitrate, that may provide a hint if there’s a big organic load hiding in the gravel)