As I said earlier, I had an ornament in my tank and I was also having ammonia problems... bought it at the local low cost chain store that also has a pet department. You get to guess which chain it was. It was the only thing in the tank other than water and two fish. I am tossing it, as I think it was adding to my ammonia level somehow. As I said, I can not prove this, and I was not going to use it again to see if it would repeat the high ammonia. It isn't worth that to me, however, now I am really getting curious about that. I may have to run a test to see what happens.
I'd try removing your decorations. It isn't going to hurt anything, and it just might help. Please turn them upside down and give them the sniff test. I'd love to know if they smell strongly of fresh plastic resins.
Now you have me really curious about the above. When I get my big tank set up and the fish moved, I will give that darn decoration the big test in my smaller tank by setting it up for a new cycle, and report my findings. It will be after the first of the year before there is anything to report.
Gold Fin