Leaves weak and crumbly

Purpling of leaves means temperatures dropping below 67, manganese will start to have real trouble uptaking from rootzone if temps are below 64, attempting to bring out the anthocyanins at the wrong time in flower can be detrimental in the healthy development of flowers. Also highly suggest you get your RH below 65%, there is often a 20-30 RH difference from top to bottom in a grow tent, make sure temperature/humidity are measured at both canopy and rootzone if possible, anything over 65% will start to close stomata and greatly effect plant respiration also. Especially during bud formation, humidity should be kept neat around 40 if possible.