I agree with kc. A HOB fuge would be a good route to go, to help increase your pod population (which your mandarin will appreciate), and help keep nutrients in check.
I do think you're overstocked, have some questionable choices in your livestock, and you'd probably be best served to slow down.
That said, if the "Noob" board was like this back when I first started out, I doubt I would have stuck around here for long. Some of the advice given is being delivered in the wrong way, and TBH some of the advice given is just wrong.
I've never kept a goniopora, so take this FWIW, but 3ppm NO3 is fine. In fact, I've found that my SPS do better with a little NO3 in the water. Strip away all of the nutrients, and colors pale...but with a small amount of NO3 (I've found that between 5-7ish ppm is the sweet spot) things look much better. Of course the trick is getting just the right amount, and it can quickly end up being too much.
In general LPS do better with more nutrients, I would assume goniopora is the same, but as I said I have no experience with those corals. Personally, I wouldn't worry about 3ppm of NO3.