How do you breed fish on fantasy aquarium for ds?

Unfortunately, you can't breed fish in the game Fantasy Aquarium for DS. While the game has a wide variety of fish to collect and care for, it doesn't offer any breeding mechanics.

The game's focus is on collecting, decorating, and enjoying your virtual aquarium.

Perhaps in a future update or sequel, the developers might add a breeding system, but for now, it's not a feature.