Hi.. My name is Fred. I have 5 different size tanks loaded with plants. I read your post.
Don't give up.One of my tanks have Caribsea substrate(peace river)but I didn't think that was enough. So I just added a bag or 2 of Fluorite underneath the carib sea, I think that will solve your problems. Plants absorb 80% of there nutrition thru there roots.go to my page and look at my 20 gallon tank. Thats the one i did this 2. Let me know if this works... Good luck!!