I have been testing my tank for ammonia and it has been at 2.0 ppm for the last few days. The hornwort will not pull all of the ammonia out of the column.
Is the hornwort newly added? Plants can go through shock the same as fish if the water's very different from what it's used to or if you don't have enough light. How many lumens do you have over the hornwort? Light is needed to drive photosynthesis.
If the plant were actively photosynthesizing, it could remove 2.0 ppm ammonia in 4 hours, just look at table 2 here: http://www.theaquari...ical_Filtration
Also, 2.0 is weird. How long ago was your test kit first opened? Has it passed its expiration date printed on the bottle? How long after testing did you read the results? Want to post the picture here?
Why is 2.0 weird? Because it's high. Ammonia didn't hardly reach 2.0 on its peak day in this uncycled tank demonstration:
And by the way if you want bacteria, API Quick Start is the product to buy. It's nitrosomonas and nitrobacter bacteria alive in a bottle, like how yogurt keeps bacteria alive for a while in a sealed container. Add it and your tank is instantly cycled.
Are you adding fish flakes or any other fish food to this tank? How frequently and how much?
"The bacteria is what matters."
Plants are just as capable at removing nitrogen as bacteria, and to disregard their abilities is a mistake. I can post pictures here of nearly a dozen tanks I have set up currently and in the past that function on plant filtration. Here's just one, my current elassoma okefenokee 10 gallon tank that I'm looking at as I type this. It's got an air stone, some hornwort, and a 1600 lumen full daylight spectrum compact fluorescent bulb.