Brown spots! Any ideas?

Hey, i've 2 big buddha critical mass autos on the go and a bag seed. The autos are about 3weeks old and i've only fed them once with canna vegga. I planted them in enrinched potting mix that says i shouldn't need to feed for 3 months. Anyway i noticed that the tips of the leaves are pointing down and one of my girls has developed some brown spots. I has the same problem on my first grow and the leaves ended up dying. Any ideas as to what the hell i'm doing wrong?

here is a chart that might help. I ran into brown spots...added calmag and it stopped. View attachment 2125988

Thanks, this points to a potassium deficiency which could make sense considering the autos are starting to flower, will give it bloom nutes next water and see what happens.

potassium def. is yellow.
you are running them too rich, imo. they are in hot soil and you fed them. N abundance.

Ok thanks, i'll not feed at all then.