Paph. Leaf Tip Die-Back

[aerides]

I was having leaf-tip die-back with several paphs when I was growing in NYC. I potted paphs in straight sphagnum and used NYC tap water, which is generally considered very acceptable for orchids. The light was HID but the paphs were off to the side. There was fogger in the room that kept humidity at good levels. Probably was using 20-20-20 at 1/4 strength as a constant feed, monthly flushing with clear water. I remember hearing something about fertilizer salts bonding with sphagnum so they can't be flushed but don't know about this. I can't remember exactly which paph this was, possibly delenatii or a cross. I'm back into orchids at a different location so naturally I want to get paphs! It seems a good time to begin working on insight into this issue. Thanks for any help.