Ideas

Hmmmm. I'd go for a leaf fish, with lots of floating plants, and some guppies for food. :) Of course, in my own 20, I have a single clawed frog. :D
 
well what are you interested in? there are plenty of interesting fish no matter how you set it up. personally i like gouramis for freshwater, and puffers for brackish. of course you could always set up a nano reef
 
That's the charm of a leaf fish--there are several species that will be fine in a 20.
 
what not try a fast flowing temperate (chinese?) system? You could have white clouds (best seen at lower temps and with water movement), chinese hillstream loaches etc.

You would need a couple of powerheads in your tank - you can disguise these with lengths of state.

Perhaps an airpump and fast moving bubbles via an airstone would also help re-create the chinese river?
 
For a 20 gallon, I'd recomend to discard the saltwater option.

For what I've learned, saltwater chemistry is more difficult to mantain that of a freswater tank. Having this small tank it would be rather fast changing, and due to the cost of the equipment and price of the fishes, I's rather use at least a 50-70 gallon tank for marine.

Those were my two cents. Let ne know if I'm wrong. Thanks.
 
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