I've used both semi-hydro and passive hydroponics, often called hydroculture, using a system similar this this:
http://www.hydrocenter-kw.com/images/luwasa.gif
This system uses a float to show how much solution is in the reservoir, and recommends to just top-up the solution to the recommended level.
I've found after about a year, the hydroculture plants start to decline because if you don't always dump the existing solution and flush the LECA, it results in salt buildup that damages the roots.
Semi-hydro works much better in the long run simply because of the 2 holes in the side that force you to flush and replenish with fresh solution with each watering.
Cheers.
Jim