The main thing is stocking - don't overstock and it won't be hard to keep water quality in check. The only thing I've found is that if say, soap gets in a tank, a little amount of soap will have a much greater effect in a nano tank than it will in a larger tank. I have had some experience with this, one time I accidentally rinsed sand in a soapy bucket. I used the sand in my 55g and my 5g. All the 55's inhabitants were fine, but half of the 5's inhabitants died. (They would have all died had I not taken them out when I did) The main thing with smaller tanks is to just not overstock - don't do that, keep a good pwc schedule, and you'll be fine. Saltwater on the other hand, is a whole other story