Why have the leaves on my plant turned pale and crispy?

Oh, And if that's the case, do a full reservoir change with a regular feeding.

I believe both of these answers are right for the wrong reasons. It looks like your running a DWC or RDWC and the plants roots are right in water. It looks to me like PH fluctuation. If your PH ever goes out of the 5.5-6.5 range your plant will show deficiencies. This is not because you did not provide the nutrient's, Its because the plant could not absorb them. Even 2 hours out of range can do this. Especially when lights are on and its hot.

Looks like a cal mag deficiency! Flush your soil with a lot of good water and wait new grow looking good, then you can fertilize again, remember, specially with autos, give less nuts! For autos I give only P in late stage flowering, if the soil is good the nuts put in it are enough! Good luck and check my diary if you want ;)