1st Grow ? Leaf problems

Heeellllppp!! using 20 litre Ikon oxypot however only using 13 litres of water (was told to do this), sensi grow 2 part nute, pk 13-14, big bud finished,(end of week 4) now in week 5 (+4 days) of flower. Was promised help and have been left to it , aint got a clue whats going on with these leaves! pH is sitting between 5.5 and 5.8. Not sure about ppm but definately not over 1000ppm. Have a look at pics.....please....need some feedback asap!!!

Change the res and get a tds meter.

Sorry forgot to say..Res is changed on a weekly basis, have an ec pen but not sure how to use it.

Hmmm........ I think it may be a phosphorous deficiency caused by a low ph. Get it up to 6.0 and watch. Learn how to use your EC meter.

Cheers, will do m8, only got the EC pen and pH up yesterday, need a buffer solution for the EC, will be getting it 2moro.