Tank Size~
Due to their small size, Endlers can be kept in a small of water. 5 of these little fish can easily be kept in a 3 gallon tank, with proper filtration and tank maintenance. In a tank this small, I would recommend an all-male group. Males usually top off at just under an inch, and females at about 1 1/2 - 2 inches.
Feeding~
Endlers seem to be rather accepting of any type of food. My guys really liked Hikari Fancy Guppy Pellets, and Brine Shrimp. High protein diets really bring out colors, even in the almost-colorless females. They have very small mouths, so make sure they are fed a small food.
Tank Mates~
Endlers are very peaceful, and quite small. They should not be kept with fish that can eat them, or bully them. I recommend smaller Tetras, such as Neons and Glowlights and Embers, Celestial Pearl Danios, Pencilfish, Bettas, ect. They are really fast little fish, so they need to be kept with fish that can easily keep up with their fast pace at feeding time.
These fish are related to Guppies, and thus, they can interbreed. In my opinion, they are not to be kept with Guppies, as its very hard to find pure Endlers these days.
Decor~
Plants are a must with these little guys. They love to swim through thick patches of plants, and play between the stems. Females also need a place to get away from the ever-pestering males, and a heavily planted tank will help with that. A more natural looking tank will really bring out these little guy's colors. I, personally, recommend a black gravel or sand, live plants, and some driftwood.
