My 40g tank has been running for about six months now. It's dirted, t5ho lights 80 watts roughly 24 inches off the substrate. I dose Seachem Iron, Flourish, and Potassium 2x a week. I also do a 50 percent waterchanges weekly. I have swords, java fern, crypts, Anubias, and dwarf sag. The plants are growing, not super fast but not slow either. One thing I have noticed is they swords in particular just don't look very lush, even the new leaves it grows are dull. They actually are very veiny looking leaves. I tested for nitrates today, which I usually never do, and it came up zero ppm. The tank is lightly stocked with 8 xiphophorus mayae swordtails. I'm thinking I need to dose Nitrogen, would that be correct? Are my plants starving?