The way I look at it .... in the tropics, daylight is darn near perfectly 12 hours any day of the year. So my tanks are lights on for 12, lights off for 12.
50g: halides for 12 hours
75g: halides for 10 hours, VHO for 12 (currently no VHO thanks to the zap!, however, but will return to the 12 hour cycle when I have time to look at it)
20g: halide for 10 hours, no dusk or dawn, but gets residual light from other tanks/plant stands/etc.
But you already knew all this, so I'm just wasting electrons by typing it in.
