I keep a breeding colony of guppies in a 10 US gallon tank. That means about 30 litres of water after you account for the substrate. I don't imagine a slightly smaller volume would even be noticed. In my breeding colony, only about half the space in the tank is available for swimming for the adults. The rest of the tank is packed with java moss to save the fry. My endler tanks are far larger because they fill a small tank so quickly.