[Eugene]

Heterandria formosa. Seems to be ideal "nano-pet".
And having a couple you will see absolutely natural thing: adult and young fish together. I have a group in 7 liters. The only care - to feed and give to somebody some fish, when there are too lot of them.

Another good fish for nano - Endler's guppi. But it's bigger and need bigger tank.

 

[Eugene]

In addition.

7-l tank with formosoes, shrimp and snail (with soil-containing substrate):

also exist yellow (xsanto) form of formosa.
Sorry for poor foto (I'm not experienced in it). The tank has no heating, small aerolift filter, 15 watt compact fluorescent reflector bulb (for standart screw, with electronic inside) 2 700 k, 12 hours. Water changes - about a cup per week, twice a year filter washing. From time to time - cutting grass and taking off fry (when it is too much). Mainly life food (dry from time to time).

One more nice "nano-citizen" - dwarf badis

Hardy, shy and interesting fish, for wich 10 l. tank is enough for the group. Easy to breed. Take care on youngs in common tank.