Without knowing more hard to say.
A list of some helpful information would be
-Are they from seed or clone
-What stage in their grow cycle are they in
-Knowing what kind of nute regimen you have them on
-Do you balance your water's PH if so what to? along with your soils PH (It kind of looks like soil is your medium)
-What are their light conditions
-What are the grow space conditions (size, air ventilation, ambient humidity, air temperature)
-Has any stressing factor happened to then recently? (bugs, light stress, heat, cold, ect...)
-What section from the plant did you take those affected leaves from