It sounds like you're experiencing a few issues with your cucumber plants. Here are some potential causes and solutions:

1. Leggy Plants: This is often a sign that your plants are not getting enough light. They stretch towards the light source, resulting in long, thin, weak stems. You mentioned that your plants have reached the light, which is 22 inches from the net cups. This distance might be too far, causing the plants to stretch. Try reducing the distance between the light source and the plants.
2. Drying Leaves: This could be due to a number of factors, including overwatering, underwatering, nutrient deficiency, or a problem with the pH level. You mentioned that you adjusted the EC (Electrical Conductivity) by adding more nutrients, which seemed to help. This suggests that nutrient deficiency might have been the issue. Continue monitoring the plants and adjust the nutrient levels as necessary.
3. Yellow Veins on Leaves: This is often a sign of nutrient deficiency, specifically a lack of iron, magnesium, or manganese. These nutrients are essential for the process of chlorophyll production, which gives the leaves their green color. If these nutrients are lacking, the leaves can turn yellow. Adjusting the nutrient solution to ensure that these nutrients are present can help.
4. pH Level: The optimal pH level for most hydroponic systems is between 5.5 and 6.5. Your pH level of 5.5 is at the lower end of this range. If the pH level is too low, it can prevent the plants from absorbing certain nutrients, even if they are present in the solution. Try raising the pH level slightly and see if this helps.
5. Overcrowding: You mentioned that you have six cucumber plants in an 18-gallon bucket. Depending on the size of the plants, this might be too crowded, which can cause the plants to compete for nutrients and light. If possible, consider moving some of the plants to another bucket.

Remember, it's important to make changes gradually and monitor the plants closely to see how they respond. It might take some time to find the right balance of light, nutrients, and pH level for your specific setup.

If you'd like, I can find some resources that provide more detailed information on managing hydroponic systems. Would you be interested in that?