How big is this tang? I know they say "Too small of a tank" and I agree to some extent. IME all my fish (if I got them small) grew to the size of their environment. I have never had/seen a fish (fresh and salt) outgrow their tank. I've had upwards of 12 tanks in my house at one point. So if the tang is little I would say he is okay for the time being. As for your question. Some will say your tank is overstocked, this may be so BUT many reefers have overstocked tanks. A "overstocked" tank just requires more husbandry than others. With that being said, I like to run a BioPellet reactor and refugium. Both are great NO3 and PO4 removal and cost effective. I set up my BioPellet reactor for $57. Regardless I had a pump laying around. I have both a 20g tall in the early stages of a full anemone tank and a 65 Corner reef. Both have very good levels and will be tied together within the near future thanks to those 2 nutrients exports.