What do you mean by Water: 5 gallons of RO water that is around 300 PPM before I ad nutes ?RO water ppm is usually well under 10 ppm.

If you are using such dilute nutrients and if by: weak 12.5 ml of each nutrient and then three weeks later which was also very weak, you mean you are using about two teaspoons of Grow, Bloom and Micro (total 6 teaspoons) as the total nutrient in your 5 gallon solution, then your plants are nutrient starving and the older leave might be dying as the plants send certain limiting nutrients from the old growth to the young ones.

At this point, I would do something like:

Grow: 12.5 ml *per gallon* (63 ml = 5 gal)
Micro: 6.25 ml *per gallon* (32 ml = 5 gal)

No way to get around feeding your kids if you want them to grow big strong and nutritious, but two weeks ought to be fine and three weeks if you get lazy. The plant needs a minimum nutrient strength to start pumping nutrients now that they are out of the seedling stage. If you really are using RO water, you will need to supplement with Calcium and Magnesium though. But that is not clear. Better to just use two days of sun tanned tap water (to drive out reactive chlorination as best as you can) with a "ppm" of around 150-300.

Then, put a computer fan or two in there so there is a slight breeze to increase transpiration. Basil likes warm temperatures and transpires more in them. You have a relatively cool environment. For that reason nutes will need to be even stronger so the plant can get its dose. I think you have a good setup and the plants ought to be healthy, but you are running into a combination of these problems of nutrient deficiencies.

I agree RO water should be much lower, 10 ppm if the filter is working well, and about 50 ppm when the membrane is getting old. My tap water is about 250 ppm. I don't even use an RO filter any more, though, I use fish tank water instead.

Leaf curl is a generic sign of stress, and it can thus be caused by a lot of different problems.

If all else fails, try the fish water. Fish poop is great stuff. Once you get the bacteria going well in your system, the plants will just take off. I have had systems just like yours that were going great, but between crops I scrubbed everything clean inside the reservoir, and the next plants were immediately sick, stressed, and nutrient-starved. What had happened is that I had killed all of my good bacteria that are responsible for nutrient uptake. Starting off with a new system is similar. It will get better with time. The fish water speeds up the bacteria-building process, though.

Do you have an aerator in there?