how hot is your grow room? do you keep the rez. temp pretty cool or does it get hot? anyways about your rez. you should change it every 2 weeks. every week is better but with that size 2 weeks max. did you lift the pots out of the buckets to check the roots? how do they look? you do have a tds meter or is that a guess?? its true that lack of calcium hurt the plant but ive never seen it make plant tip leaves curl down like that. what usually happens is you get brown spots that dry up and crust away and the tips of the leaves get a very little bit yellowed or burnt, and while in flowering it can cause the plants stalk to be very flimsy. i honestly think there is a root issue going on. oh btw what is your pH???