Since I outgrew taking electrical outlets apart by age ten, I've been playing with other things. Right now, it's Kratky jars, and they're a bit more challenging than I expected. Contrary to what I've read on the Internet, they are not set it and forget it projects. They, too, have their quirks. Please allow me to explain. 

Quirky Kratky jars.

I made my Kratkys from 1/2 gallon Kerr jars. Those accept a 3" net pot very readily, and the jar ring works well to secure the pot. It's important that roots have some air, so they don't rot, especially in these. I had a nice micro-tomato in my other unit, so I transplanted it to see how it would grow in the jar. And it grew very well until one day, I noticed that the nutrient solution in the jar was very low.

The tomato had set fruit but was a long way from ripening, so I added more solution. The tomato wilted and died within a few days. Upon doing more in-depth research, I learned that this is actually a quirk of the Kratky method. 

Lesson:

When growing something Kratky style, it's important to grow something that will be mature and harvestable by the time the nutrient solution is used up. Adding more is NOT an option, at least not in the traditional way. 

Solution:

Take the plant completely out of the unit. Clean the jar with some bleach, refill it with the nutrient solution, and replace the plant. I have no explanation for why this works, but it does. The micro-tomato I'm currently working with didn't die and is now setting fruit.