It might be coming from the substrate, did you wash it thoroughly prior to adding it to the tank?
Personally I always use fishless cycling with pure ammonia, I would do fish in cycling if the tank was going to be planted.
Plants are extremely good at taking out ammonia, nitrites.
Am not questioning Byron in any way but I would rather not cause the fish to go through the cycling process even hardy fish. Long term ammonia and nitrite reduce their lifespan. Done right though and slowly it can work with good effect.
With pure ammonia as well, high dosing ammonia creates a massive amount of bacteria, likely more than you will ever produce from the fish alone. I fully stocked my 75g tank with all the fish after cycling, all fish added the same day, there was never any ammonia spike at all. Never has been in the tank.