This tank was cycled before?
The change in biofilter or bioload could also cause an ammonia problem or mini-cycle.
Other things like dead material on the bottom could cause this. Do you do 25% substrate cleaning with regular maintenance?
Small water changes (ie. 10%) could help this, but watch those other params.

Amquel detoxifies ammonia, meaning that the product bonds to the free ammonia (the bad stuff and the stuff that the salicylate tester measures) and makes it unharmful to fish. However, it's still in the aquarium for the good guy bacteria to use and turn to nitrite, then nitrate. However, it shouldn't be used as a cure. You'll still need to do water changes. You could add a bit extra with your next change, but keep measuring for a few days and see if things even out.

As for plants helping, it's my understanding that plants first use nitrate, then nitrite, then ammonia (and less of each as you continue down the chart).

~Ash