Doesn't look like overwatering to me. I'd venture to guess there is a hot spot (high nute content) in your soil. You've got what is known as the rams horn showing in the leaves. That is why it's so important to mix the shit out of organic mixes, twice. Being the northern lights is the only one effected, and we know you havn't been overfeeding them , that is very likely the problem. To fix it give plain ph'd water to the N. lights for a day or three (watch the plants reaction, it will tell you when it's hungry), and continue to feed the rest bloom formula. Plants look great otherwise, good job bro.