How long are the fish?
They are a bit young to tell but there is definitely 1 male and possibly several females.
The first dorsal (top) fin on most good quality male rainbowfish will overlap the second dorsal fin. This does not happen in females.
The anal fins on good quality males will be longer and more pointed than the females.
Males get higher in the body and females stay much narrower in height.
Male rainbowfish produce a dorsal blaze when displaying. This is a white, yellow or pink stripe that runs from the first dorsal fin to the nose. Females do not produce this. The stripe only appears when the males are showing off and is a different colour for different species. For melanotaenia lacustris (your fish), the dorsal blaze should be yellow.
You will need a bigger tank when the fish mature. They need a tank that is at least 3 (preferably 4+) feet long. You need to have coverglass on the tank because rainbowfish jump.
They need lots of plant matter in their diet. At least 50% of their diet should be plant based. You can use vege flakes/ pellets, goldfish food, Duckweed (small floating plant), or try pumpkin, zucchini, etc.
