Do you have a flush day where you just give them water? One or two days a week you should flush with straight water until a small amount runs out the bottom.

 It's a good idea to test the ppm of the water coming out the bottom if you have an EC or TDS meter.

Also watch the "new" growth for signs of burn that indicate too much nutrient. A lush green/vert color is what you want, but not tip burn.