Ok, to dose every 6 hours, the code on the apex is:
OSC 000:00/013:30/346:30
This means it will dose for 13 minutes and 30 seconds every 6 hours, starting at 12 am.
If you add the three parameters of OSC, it should equal the total number of minutes of the total interval. So in this case it's 0 + 13 min 30 sec + 346 min 30 sec, which equals 360 minutes (6 hours).
If you want to start at say 2 am, the first parameter gets a value and the third parameter gets adjusted accordingly based on the total interval. The code will then be:
OSC 120:00/013:30/226:30
This means, it will wait for 120 minutes (2 hours) from 12 am (which is 2 am) before it starts dosing. It will dose for 13 mins 30 sec, and then wait for 226 mins and 30 sec. If you add 120 min + 13 min 30 sec + 226 min and 30 sec, it is still 360 minutes (6 hours).
I hope I didn't confuse you.
Btw, this will only work on the Apex and not on the older AquaController III. Apex supports seconds.