Your light is too close. Lift it up. General rule is, hold your hand at the top of the plants. If the light burns the back of your hand then the plants are burning too.
Give them all the ph'ed water they can use to keep rez full inbetween water changes.
May have been nute burn but if you backed off the nutes it must be the light. Some light shields can cause hot spots too.
One more thing. What strain are you growing ? I ask because if you have alot of branch growth then the main leaf can die off and the branches take over. I had it happen to papaya and another strain. No problem as long as there is plenty new growth.

Black spots sounds like mold but I don't think so. Do you see any bugs ? More info is needed too. Water temp? room temp? Hot temp and your plant will drink all the time.