Small Problem On Upper Fan Leaf.

Ok well Let me give you the story on my plant. It is about 3 weeks old I just started feeding it Biothrive Grow 4-3-3. It had a small nitrogen deficiency(that I am guessing it was) which was yellowing first leafs then going up to the first fan leaf but as soon as it got to those and hadn't yet fully yellowed the first fan leafs I started feeding. But The first day I fed I noticed a small black spot on the second fan leaf really small which then turned into a dead spot on the corner of the fan leaf not the tip but on one of the spikey sides. now right beside that today I noticed a nother black spot really small it could be brown but looks black. Can anyone tell me what this is a sign of? I am using Happy frog soil. I was watering with sink water for the first 2 weeks could that be the problem? but now since I started feeding I am using distilled water. Can some one give me some answers

Ok after reading I am thinking its a Potassium Def Because I was late to start feeding the plants they had already started showing bad nitrogen deficiency. And potassium def All plays in when not giving your plants NPK And It said lead margin turn dark and die And that seems like whats happening. And let me get this straight not feeding your plants NPK can lead to many other problems Instead of a NPK deficiency? So not feeding them just causes many problems for your plants cause if your plant has too little P then another problem evolves and so on with N and K correct?

ok it cant be that some one say something do you need a picture come on people