I wouldn't rely on those hang on ammonia readers. You should have some basic test kits to monitor your tank. PH, Ammonia, NitrAte, NitrIte, CA
are nice also.
With your tank being young as it is you will see fluctuations in your water chemistry as things mature.
What kind of macro algae (your plants) do you have in your fuge? Most if not all of us cut our macro back every so often. That is how the nutrients are actually taken completely out of your system. I trim mine ever couple of weeks.