Mother plants in soil, I think its a Phosphorus deficiency What should i do?

This is my 1st time using this site and i am finding it quite helpful. As the title suggests I have 5 mothers all about 3' in soil that appear to be showing Phosphorus deficiency, Yellowing of upper leaves then center of leaf is getting brown spots, Jorge Cervantias book, is where i came to this conclusion. I am getting water through a filter that is hooked to my water line in my utility room, could this be the cause? But clones that I have taken are getting the same water and they are not showing the same effects. Any and all suggestions will be greatly Appreciated
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hey man most ppl on here wood rather c a pic then to try to imagine wat ur describing so they can bttr help u