discoloration in center of leaf

[SENS1MILLA]

hey guys, my plants are growing smoothly put ive noticed some discoloration/yellowing in the middle of the leaf take a look at the picture and tell me if anyone knows what it could be.

thanks.

 

[Mr.Wakenbake]

that is the beginning signs of Mg def. it normally comes with other def also. looks like its time to feed em man.
wouldnt hurt to spray em down or mix some epsms in with your next feeding. ckeck your ph akso as that could be it but i doubt it .