Help! nutrients problem

[PCNinja]

For the past couple days i notice the leafs have a lot of black spots...im assuming this is Phosphorus deficiency?? whats the best way to resolve this?? I have been using DNF and used 1/2 the suggested dose....this girl takes about 500ml and i water it once every 5 days...the temp is averaging 25*c

u can follow my current grow at:
http://www.icmag.com/ic/showthread.php?t=163909

I hope does doesnt count as double posting caz not much ppl check out the diaries and i hope i post this in the right section