Brown leaf tips are a trademark sign of nute burn. What medium? what nutes? lights? how old?
It also looks like to me that your leaves are a very very dark green/purple color. This might just be a camera trick. But if they really are dark dark green/purple, it may be a phosphate deficiency. It may also be the strain, I believe green crack also has some purple kush in it?? Not positive on this one though