You said you've had it for two and a half weeks? How did you cycle it? Typically that takes a month, or so and you end up with zero ammonia and nitrites and your nitrAtes are high. Do a water change to lower those trates and you should be good to go. You do not want to add anything with 40ppm.
CUC are not necessary, unless you really like all the little critters. They do clean, but they still add to the bioload. What goes in, comes out. It does not magically disappear just because a crab or snail ate it.
Make sure that you clean that filter regularly once you get fish, as it will become a nitrate factory if waste builds up inside. Once a week, maybe more often depending in what you get.