I've had that happen. It was the pin the on one of the bulbs that was broken so the connection was bad. I would imagine that it is a connection that is loose somewhere.
I've always thought of nanos lower than around 30-35 gallons, but there really is no definition. Just a smaller sized tank. You can have it as a mini reef setup(Corals) or just a fowlr(fish only with live rock). They are harder to manage cause of the small water amounts to dilute toxins/crap...
Hey I was looking at getting a fuzzy dwarf lion. I have a 24g nano. Would this be large enough. I've seen some say like a 30g is what you would want. Let me know what you think.
here is one point of advice. Don't believe everything the lfs tells you. Some places are good and only try to do their best while others will just try to sell you crap. Always come and do research then go and buy. I've lost several fish due to bad advice given by lfs.