Some people would say that a "Nano" tank is harder to keep than a larger tank, this may be somewhat true in the sense that there is more maintenance required to keep chemical levels stable. As long as you stay on top of tank maintenance you should be good!
As far as the sand goes ... are you getting algea growth or Diamtom Blooms or Cyano
???? Just dirty really doesn't tell much, if it's cyano
your dealing with there are multiple causes and fixes ... algea and Diatoms are different in that they can more than likely be controlled with a cleanup crew and a more strict feeding routine. Please elaborate as to what it is your refering to as "Dirty".
As for Cleanup crew what do you have? I have a 12gal and I keep about 6 small Nassarius snails, 3 Scarlett hermits and 5 Turbo snails. My sand bed is not completely clean, but overall well kept. During my bi-weekly water changes I also try to syphon out the piles of detrius that accumulate in certain areas the best I can without syphoning to much sand!