What is the herb in question? Looks like arugula or something... Are there any other affected plants? Have you noticed any signs of insects or fungal damage? Is it possible the damage is caused by being too close to your lights? Have you tried the hydroponics forum here? Might be something related to pH and/or nutrients in the system. Hydro growing is generally a process of constantly monitoring and correcting water chemistry and most of the people who frequent this forum are growing old school in dirt or in potting soil :)

It's hard to say without knowing what you've been doing and what your setup is. The yellowing leaves could be a nutrient deficiency if that's what you're asking about. Pluck or cut off that one leaf with the white/dying edges.

On a related note, your plants are way too crowded. There should only be ONE plant per pot. It's counter-intuitive, but you'll get more yield that way. Otherwise, they just crowd each other out. The exception to this rule is if you're just growing them for microgreens (harvest after about 2 weeks) or for grasses (like chives).

So thin your plants out by cutting at the base with scissors. DO NOT pull them out as this will harm the root system of the plant you're keeping.

In the future, definitely plant a few seeds per pot, but just thin them when they get to about 2 inches tall or have 2 sets of "true leaves" (the ones after the first "baby leaves").

I'm not sure how relevant the advice you're getting here is. From your opening post, "Just recently I picked up the hobby of growing herbs and vegetables hydroponically "

That last word throws a big curve ball into the mix.

There's a "hydroponics forum" here, and you might get better answers there. Actually, before posting a question there, you might check search results for "yellowing leaves in hydroponics."

Those look like dandelion greens. Are you fertilizing them?

I don't see anything that looks like dandelion.

The post is a year old but fwiw the mystery plant is mizuna. So used as a salad or vegetable rather than a potherb.