lights

I have a new nano(new to me) and my white light flickers on and off. I press down hard on the light switch and it'll turn on but in less than 5 min, it's off again. I got up this morning and it didn't turn on. Is it electrical? Is it an easy fix? Should I buy a new fixture? Can you even buy a new fixture to put in there?

Most common problem is a connection point somewhere. Don't even start thinking about replacing parts and certainly not entire fixtures before checking connections.

The connections are everywhere. The connection that jwoo mentioned is only one of them. Others are the connection of the pins at the ends of the tubes and sometimes it's an internal connecting point.
Move your ear close to the fixture and listen for a buzzing/crackling/popping sound as the light turns on or goes out.
Feel free to call me as you are checking it and I'll help you isolate it.

Yup that's what I thought you had. Bad ballast is almost 95% certain. Everyone I've know who has had one (including myself) has had the ballast go. Lucky you they are cheap ($35) and easy to replace. I'd just do what I suggested above to make sure then get a new one. There are literally thousands of posts across the internet about the crappy ballast in biocubes. You can check the other connections but I'd put money that it's the ballast, Mark probably just doesn't have the experience with oceanic biocubes and know about the design flaw in these ballasts. It's pretty well documented though. Do the quick check to verify.