What soil you use and what fertilizers?Well, we are going to bet that it is a deficiency. Tonight I'll raise the EC to 1.2-1.5 to see what happens. I was fertilizing them little because less than two weeks ago I did the last transplant from 7 to 11 with fertilized substrate and I thought that this would be enough for now. And besides, I was afraid of overfertilizing. Thanks for answering @Fish
How you consider if you over water them in some point or if to hot or to intense light ?