2 weeks flowering, new growth curling under

Yes, the reservoir is 7.5 gallons; its a Hydrofarm MegaGarden grower.

I figured it would provide enough room for 4 plants, but this is my first grow so I could most definitely have calculated that decision incorrectly.

I waited until the plants were 22 inches before moving the 4 plants to the flower cycle, and was given advice by a local grow shop to give the Bushwhacker the first 2 weeks of flowering to reduce stretching.

The last reservoir change was the second week with the Bushwhacker, so I won't be using that any more.

Other than the curling leaves on top, the rest of the plant looks healthy, not to trivialize the problem though -- I desperately want to solve the problem before it totally destroys my babies!

Oh, and allow me to add the plants were vegged in their rockwool grow cubes, then moved into the megagarden grower when the bloom cycle started 13 days ago

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.