I have one diamond tetra and though I know tetras are schooling fish, can my diamond girl be happy by herself or maybe with just one more? She's active and no one picks on her and she doesn't pick on anyone.
Although its usually a better situation to keep a schooling fish in a group, there are no set rules, which dictate
an actual number. If your Diamond Tetra is eating and otherwise acting normally in the current situation really no urgent need to add others.