How about : lack of real sun. I have to put my Basil outside - in the sun - everyday or it isn't happy. ( I have no idea what the Kratky method is.)

That kind of color change (yellowing and darkening) reminds me of downy mildew, but I don't know if this applies to hydroponics (started from seed indoor?). It's rampant all over the East coast.

Check your ph. High ph can do that and usually tap water is high. A bottle of ph down will last you she's in that small of a container. The ph affects some nutrients. I forget which does that, I think nitrogen or magnesium.

Thanks for your reply Barb!

Unfortunately that basil ended up dying - and from the look of the roots, it seems the culrpit was root rot.

Not sure what caused it? What are the usual causes of root rot in hydroponic systems?

I had rinsed the roots with hydrogen peroxide and given the plant brand new nutrient solution with adjusted ph, but it didn't seem to help.

I had the same thing happen to a parsley plant I also started in a Kratky system.

However, my second parsley plant is thriving right now and the roots look great.

All plants received the same care, so I can't figure out why two ended up dying of root rot and the other is thriving.

If the hydrogen peroxide is too strong it can negatively impact the roots, if trying to treat your water/roots use a lower concentration of hydrogen peroxide or a natural conditioner like cietecbio.

Also it seems the older leaves were dying off but your new leaves looked fine. if you have exact ph and ec (ppm) values let me know.

The water level in kratky is critical. The smaller the container, the more tricky it is to stay on track.