You may need to add the other macronutrients plants need. Specifically phosphate and nitrate. Are your plants growing well?
I like to think that my plants are growing well, I already trimmed some of them because of too much growth, but Im not sure if they really are healthy since this is my first month in this hobby,
this thing is coming form my mini amazon swords
Here is a video I took around 2 weeks ago, my tank is 3 weeks old in this video(plants are planted the first day.Some of my glosso leaves has holes in them, I believe that they got shocked after I did a major trim on them,since it appeared after trimming them)
aaaaand here is a pic I took this morning(some plants already trimmed couple of times since the video.)
I should also mention that the back part of my tank doesnt grow algae, only my front part since my lighting is sitting in the front part.I just want to make sure that too much lighting are the only cost of it and not imbalance/poor water quality.
Be careful dosing those nutrients, overdosing will probably be one of your major issues, neither of those are going to show up on your test general API test kit.
I don't dose with macro's personally, but it does sound like you are overdosing your iron and potassium. Try and stick with the more comprehensive Fertilizers these are easier to balance this can be especially hard to balance in the smaller aquariums.
I am currently using Sea-chem Flourish, flourish excel and flourish Tabs and these work great for me, have had no problems with yellowing or wilting. Keep in mind substrate is important as well plants can pull macro nutrients from there, I am using Eco-complete and Fluorite mix this has worked well for me.
Could you link the info for your LED array, 38 watts of LED for a 15 gallon sounds like to much, but 1600 Lumens sounds about right. Something is not adding up in that equation.
Keep in Mind your light cycle is very important, 7-9 hours should be your goal for a photo period, too little or too much will cause problems. If you have not invested in a timer do it now.
Im not sure if I can post a foreign link in this forum but here it is anyway.
It automatically turn on and off, the blue leds turn on at 9 am to mimic morning light, at 10 am the white leds will turn on it will stay like that until 8 pm then the white leds turns off leaving the blue led on for 2 more hours to mimic evening light then completely turns off at 10 pm
Its for 60cm(15 gallon) up to 90cm(around 40 gallon) aquariums, they are the most popular aquarium sizes for a normal home in Japan.