I can't actually remember whether plants take up ammonia or ammonium
but it doesn't really matter. Because ammonia and ammonium are in an equilibrium (pH and temp determining how much is in which form), if the plants use one of them, the equilibrium restores the balance. It's the same for the filter bacteria which only use ammonia.
[Think of it like old fashioned balance scales with a pan on each side. Add some beads to each pan so that the pans are equally balanced. Then take a few beads out of the left pan. To keep the pans level some beads have to be moved over from the right pan. Then take some more from the left, and more have to be moved over from the right pan. And with the ammonia/ammonium analogy, beads are also being added to each pan as the fish excrete ammonia so the pans are never completely emptied]
pH does vary with the time of day, particularly when there are plants in the tank.
Animals and plants respire. They take in oxygen and they release carbon dioxide. They do this 24/7.
Plants photosynthesise. They take in carbon dioxide and release oxygen. But they only do this when it is light.
The net effect is that during the day the plants take in more carbon dioxide that is being released by the animals and plants, but at night more carbon dioxide is released than is taken in by the plants. There is more carbon dioxide in the water at night. Carbon dioxide dissolves in water to make carbonic acid. Therefore the water is more acidic at night and the pH is lower at night. The amount of carbon dioxide will build up during darkness so the pH will be lowest just before it gets light. As soon as it gets light (daylight or tank lights) the plants start photosynthesising, take up all the accumulated carbon dioxide and the pH rises.