@GaryE, we were both wrong
It's just because he has matured sooner and is bigger than the others and showing them he wants to be boss. When the other males get to the same size they will all spar with each other.
You want at least 8 rainbowfish in the tank and up to 20 for that size tank. But 8-12 would be a good number considering you have other fish in there. Try to have some females in the tank for the males. They show better colours and will spend most of the day showing off to each other in an attempt to impress the girls. Try to have similar numbers of males and females.
You can mix different species of rainbowfish in the same tank. Just keep species that grow to a similar size.
When the plants get taller, you can leave some tall and use it like a wall in the aquarium. He might spend his time there saying this is mine.
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Make sure they get plenty of plant matter in their diet. rainbowfish need at least half of their diet to be plant based. You can use vege flakes/ pellets, goldfish food, pumpkin, zucchini and other fruits/ vegs, and have Duckweed on the surface. I know people hate Duckweed but rainbowfish love it. You can grow it outside or in a spare tank and add some each week for the fish to eat. It has softer leaves when grown in the shade or under coverglass compared to grown in full sun.
