To be honest I believe its up to you. I have tanks that are on timers and some that the light is on 24/7 and personaly I do not see any difference. But thats just my opinion. I would imagine if you have a planted tank it would make a difference.
Unless you have a planted tank, i'd personally recommend you turn the lights off whenever you're not around and likely to look in there! ie. at work. The fish won't care one bit! You'll save on electricity, the chances of algae blooms will go down and it'll keep you're tank cooler if it's particularly hot.
Obviously plants need light to photosynthesise, but upto 12 hours is usually fine for them. In fact it is particularly important to turn the lights off at night if you have plants, as they respire at night (the opposite of photosynthesis) and they can't do this with light!
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