Ahhh... *sigh it's one of those pre fab systems. Hrm.
Well, 7 gallons for that much biomass is pretty tight and you are going to find that you have to change out the res REALLY often, like weekly- or even less!
Some strains REALLY don't like any nitrogen in flower and personally, seeing that curl, I'd be doing the following:
-Change to a bloom fert that has virtually NO nitrogen in it
-Change the res every 4 days
-Find a way to check your EC; a meter is REALLY helpful in hydro
-Next time, start with a clone mother and use that system to flower multiple rooted clones with almost ne veg time. They will finish as a single cola plant about 8-10" tall which is almost all bud and very little leaf, and be a lot easier to manage in a system that was designed for smaller plants.

Edit: Add another air stone to that res pronto, too... lots of roots and little water will lead to problems before you know it, and the air stone will help prevent that.