Need a little help, im new!

[Getchucked88]

Hello, first time grower.. Using an autoflower strain, have 7 gallon bin that i fill with 4 gallons of water, I have two air stones with a drop feed system.. I'm using Advsnced nutrients ph perfect 3 part system.
My plants are curling amd becoming discoloured, any idea as to why?

Ph is 6
Ppms is 250


I attached 2 photos. Thanks in advanced!@