Just got my first fish tank and wanted some ideas of what to keep, only problem is the tank is only 21 litres so im know i'm pretty limited on what I can keep, i was thinking of a puffer fish, what do you guys suggest?

THanks for the help just had a look at the ten gallon fish, the shell-dwelling cichlids look really intersting how easy is it to care for these and are they easily available throughout england?
However shellies are special: Omnivorous: And they love shrimp pellets, brine shimp(you should see them stalk), daphnia, not to mention the odd algal wafer or spirulana.
The are the most interesting small fish ive ever seen, loads of character.