To work with ammonia, you need very fast growing plants like elodea or even duckweed. Plants that are dying are making the ammonia problem worse, not better. Decaying plant matter produces, guess what, ammonia. The pretty plants that we use to decorate a tank generally do not grow fast enough to help much with ammonia. In the meantime, large water changes are your friend. I am having some trouble with one of my 10 gallon tanks and I am giving it water changes by draining until there is barely enough water for the fish to swim in and then refilling daily. The fish always look better right after the change than right before.