Depends on a lot of stuff. Has the LR been cured? I would get everything set up and cycling, then using a good test kit test the water everyday looking for a spike in ammonia, nitrates and nitrites. Once everything has spiked then fallen back down to undetectable levels it is ok to SLOWLY add livestock, either fish inverts or coral. You will usually get a small "mini cycle" after adding fish because of the extra waste, but if you do it slowly everything should be fine. I would resist putting more than one or two small fish in a 12g nano. The less bioload the easier the tank will be to maintain. I am working on a 12g nanocube myself and am considering going fishless, I think the inverts are more fun to watch anyways. Good luck an feel free to ask more questions.
BTW you do not have to run the lights during the cycle, it will cut down on algae blooms. I do anyways and it doesn't seem to hurt that much.
