i know of people using these in there vivariums for fly traps and plants. also a lot with hi humidity situations like frogs. in the directions they were given they were told to put the hydro balls 2in deep in the bottom of the tank. Cover with a screen, then put the substrate over top the screen. the balls with soak up the excess water from misting or fogging and then naturally release humidity over time. with this in a plant type setting they need to be replaced after about 5 years. hope this helped.
They are in my frogs viv. Someone did the viv for me so it looks like they used some sorta floss between the ball and soil. They have worked great. It's been about 4 years with no problems.