Nutrient burn?

This picture is normally shown on heat<<// lightsteress,,, but 21 degree wont be to warm look for your feedingsolution, my be a sign for rootproblem too ( look for th ´e temperature of the solution) and if you think you overfert. delute your feedingsolution. You have to check every factor of your grow to find out what is happening. Atm iam growing one plant that shows a similar picture, and its either to warm or to bright, she just tends to folding the edges. But believe in your intuition and iam sur you find the reason happy finding

Try mixing 2 tablespoons of epsom salt in a 1 gallon milk jug and spray the plants a few minutes BEFORE lights out. The light will burn the plants if you dont. i think its magnesium deficiency