Interveinal chlorosis - could it be iron? Potassium? 3rd week in flower

Well im 3weeks in with my 5ft tall power kush but now all my leaves are getting that interveinal chlorosis shit where the leaf yellows but vein doesnt. It began with oldest leaves.so I didint think much of it at first but now theyre all begginig to do it. Want sure if anyone can help I know its a long shot without pics but hey maybe someone knows cause I sure as hell dont

Could be K or mg, get some better quality nutes and medium.

get some pics up