The nutrient is kept at an EC (Electrical Conductivity) level of 2000 and a pH of 6.8 for Basil suggested by :
http://www.aquaponics.com/articleherbs.htm

Thanks, i'm thinking 6.8's perfect too.

In the summer you may need to drop the EC to 1.6.

Thanks Adriana, what's the reason for droping the EC to 1.6? Please expand on that if you can.

In the summer the plant's rate of evaporation goes up, so to make up for this increased water need you basically water down the solution to avoid it being too concentrated. Sometimes in hot weather you can check the EC coming in and going out of the grow beds and you will find that the ec coming out of the beds is higher then the ec going in. This is a sign that they are taking nothing but water. A sign that the ec is too high in my basil is the leaves begin to get a crisp almost crunchy gnarly appearance.