Ozone will not harm anything if administered correctly.....Ozone makers should not, be put on timers....That is a definite way to have a major problem on your hand....There is no way for you to know when the ORP reading will change or go down other than utilizing a ORP meter......The meter determines when the ozone should be kicked in and out depending on your high and low settings........
Ozone can be run continuously on a ORP meter, and there isn't any need to turn it on and off......The controller does that for you, and don't recommend running them without some type of controller, be it PinPoint, Milwalkee, Neptune, Octopuss, etc.....
There is a chart of some sort as to what your readings should and shouldn't be, but I don't have it memorized, but your ORP will tell you alot of things in regards to the condition of your system as well......Just adding an airpump to your sump will increase your ORP reading.......I did experiment with that years ago to see if it was true
Readings around 380 and up are considered on the high side and when you approach that point you would seriously need to start checking out your controller settings or your probe calibration.....Around 390 and up things can become lethal......Basically at about that point your tank would start to become sterile.......