The Bluefin killifish will swim at all levels of the aquarium, not just the top like many other species of Killi. Bluefins are sexable when they are mature as the juveniles and females look the same. The males will have bright blue marks on the dorsal fins when mature, whereas the females and youngsters only clear dorsal and anal fins with no color at all. The males also have red or yellow marks on caudal and anal fins, they and are larger and more slender than females which are smaller but plumper. You can improve their color naturally by using New Life Spectrum Ultra Red food. Bluefin killies are onmivores. Bluefins prefer munching on inverts such as mosquito larva, pest snails, etc. Bluefins also will pick on the algae on aquatic plants which is a trememdous help in many aquariums. A good supplement food for them is bloodworms. Breeding is actually easy, as long as you have lots of plants such as Java moss or other mosses.
Bluefin killifish do better in heavily planted tanks opposed to bare tank or not enough plants. They will behave naturally in the planted tank. They are better in a species tank, as they can be shy or playful depending on the tankmates. Dwarf livebearers (Heterandria formosa) and Japanese Rice fish (Oryzias latipes) are excellent killi tankmates all of which do well in coldwater outdoors, as well as it is unnecessary to heat their water if they are in an aquarium.
Bluefins are a hobbyist favorite killifish because they have playful personalities and are very easy to take care of. Enjoy!