There is some serious misinformation here, I'll come to that but first comment on this ammonia of 8 ppm.
This is not at all likely, as the fish would be dead if the ammonia really was that high. However, there is an exception, and that is if the pH is below 7. Ammonia is primarily ammonium in acidic water, and ammonium is basically harmless. However, it should not show above zero, especially with live plants. This begs the question, did you do the ammonia test correctly, or is the test not reliable for some reason? Or is there something causing this we do not know about?
To the misinformation...aquatic plants we have in our aquariums take up ammonia/ammonium as their preferred source of nitrogen. They do not take up nitrates, unless the ammonia/ammonium is insufficient in balance with other nutrients and light intensity. This is because plants must change the nitrate back into ammonium in order to use it, and that takes considerable energy so plants avoid it unless absolutely necessary.
Fast growing plants--and here surface plants are the fastest--can take up a considerable amount of ammonia/ammonium; surface plants are frequently termed "ammonia sinks" for this very reason. Unless the tank is way overstocked, the fish will never produce more ammonia than the plants will remove--again depending upon the numbers of plants and the species. Slow-growers like Java Fern will remove much less ammonia that faster-growing plants. Plants take up the ammonia/ammonium day and night, so it is a continual process.
All plants use the Nitrogen from ammonium—not nitrates—to produce their amino acids and proteins. And plants can store the ammonia/ammonium. I was in a discussion with Tom Barr a few years ago, asking about the limits if any, and he said it would be virtually impossible to add so many fish to a planted tank that the plants could not rapidly deal with the level of ammonia. Again, it depends upon the plant species and numbers, but you are not going to see ammonia in tanks with sufficient fast-growing plants.
