Discoloration on one plants leaves. tips curled do...

first of all,your grow looks really good,nothing to worry about too much for now.Stats are cool, 6 ph is OK, watering every 2/3 day is OK and you have perlites in your coco it is very OK too.With coco,overwatering with every other day watering is nearly impossible,yours are not overwatered.With coco/perlite it is quite impossible.Yours just have too much nutrients in growing medium.Those rusty spots generally related to calcium deficiency but since you also think that you overfed some until now,rusty spots will turn into crispy leaf parts slowly.Leaves have a solid dark color and very little signs of burn on the edges means yes you have overfed a bit but nothing to worry about as they all look healthy.As far as I know,gorilla strains are sativa dominant ones with fussy genetics.sativa leaning plants don't like much Nitrogen.That's why you see leaf tips curling down.this clawing is caused by overnitrogen,meaning nitrogen toxicity.But as I said,nothing to worry about because you have just noticed it and did a great job! All my sativas had this clawing,I treated them quick and it didnt have affect the yield. 3lt pots are too small,I suppose you'll transfer them into bigger pots soon.My advice ; do a flush with 5lt of water.check the EC of the runoff from starts.You'll see it decreases with more water.that's good.when you have more clear runoff with less EC,you are safe.Then feed them just like you fed until now.but add less nutrients please.Also,a critical advice , that VeloKelp thing seems to be some seaweed extract.They also have too much Nitrogen in them.Giving grow nutrients alongside things like kelp causes that Nitrogen toxicity.Use only kelp or grow for your plants as they seem to be not loving it that much.I would cut off grow nutrient.only kelp,no remo grow.and you'll have all your problems got away.this one is a well prepared,nice grow.good luck mate