The serpae rule: more fish, fewer problems
Serpae tetras carry a red body, a black shoulder mark and an outsized reputation. The truth is conditional: kept in twos and threes they are restless nippers, but in a proper school their sparring is absorbed by the group. Six is the floor; eight to ten in a twenty-gallon-long tank is where serpaes turn into what keepers actually want — a flickering red shoal too busy with itself to bother anyone else. Every addition to the school dilutes the attention any single fish receives, and the nipping habit starves.
Their size, a little under two inches, makes a ten-fish school affordable in both money and bioload, and it should be bought as a school from the start rather than built fish by fish.
Behaviour in a full school
Inside the group there is a visible pecking order, maintained with short chases and fin-spread displays, mostly in the morning. Females are plumper and slightly paler; males hold the deeper red. The school works the middle level, drifting through plant stems and regrouping in open water, and a startled group compresses into a single red flash before fanning out again — the movement that makes them a display fish rather than merely a red one.
Warm water at the mid-to-upper tropical range, soft to moderately hard, with tannin-stained dimness from wood or leaves flatters both their colour and their confidence.
Tank mates under the serpae rule
Even a well-grouped school should not be housed with slow, long-finned fish — the temptation is bred into them. Choose tank mates that are quick, short-finned and similarly sized: other robust tetras, barbs, danios and corydoras below. Plants should include fine-leaved thickets where a chased fish can vanish, and feeding should be spread across the surface so competition stays brief.
If nipping appears despite a full group, check the usual accelerants before rehoming anyone: skipped meals, a school quietly reduced by losses, or a new tank mate whose fins read as food. Restoring the count fixes more cases than any product on the shelf.
Feeding style influences serpae behaviour more than most keepers expect. A school fed once a day in a single spot spends the hungry hours prospecting, and fins look like prospects. Two smaller feeds, scattered so the whole surface ripples at once, shorten the competitive window and leave the group cruising instead of hunting. Frozen foods offered in a gentle current are especially good theatre: the school works the drift together, and the energy that might have gone into a tank mate's tail goes into the chase for dinner.
Troubleshooting and topping up
Count serpaes monthly, because the school that slips from nine to five changes character without changing address. Replace losses in threes, matching size, and add them after lights-out or during a feed. When a behaviour puzzle does not match the usual pattern, talking it through early — what changed, when the nipping started, who is targeted — is how small problems stay small. For anything outside that usual pattern, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.