Iron, Sulfer or what kind of deficiency is it?

[LoIQ2]

DWC Hydro - White Widow on AN PH Perfect Nutes. PPM 675, Ph 5.9 almost 4 weeks flower. Top leaves look like pic today. Please help with this problem as guides are not clear if this is Phospherus

 

[Hotboxbudz]

Pictures of the whole plant would help us give a better diagnosis.

 

[Guy2HIGH]

Pictures of the whole plant would help us give a better diagnosis.

This is true, a pic of the whole plant is important because it shows all of the problem that the plant is experiencing. Different deficiencies affect different parts of the plant in different ways.
The pic you showed is a definite iron deficiency... you can tell for sure by the paleness of the leaf while the veins remain dark green. Iron is difficult for the plant to absorb and moves slowly inside the plant. Potassium and iron go together... too little iron usually means too much potassium and vice versa.