Numamutsu

Where they come from

Numamutsu (Nipponocypris sieboldii) is a Japanese cyprinid you will see referenced in the context of rivers and streams in Japan. Think clear-ish freshwater with current, seasonal temperature swings, and lots of natural foods drifting by. That stream-fish background explains most of what they want in the aquarium: room to swim, high oxygen, and clean water.

Setting up their tank

Give them space first. They are active, midwater swimmers and they look best when they can cruise in a group. If you keep them in a cramped tank, they get skittish and you will see more nipping and frantic dashing.

• Tank size: I would not bother with anything under 30 gallons for a small group. Bigger is noticeably easier.
• Group size: 6+ is where they start acting natural. 10+ is even better if your tank can handle it.
• Flow and oxygen: a strong filter, a powerhead, or a spray bar aimed along the length of the tank helps a lot.
• Temperature: they handle cool to moderate temps well. I run them unheated in a room-temp setup most of the year.
• Hardscape: smooth gravel or sand, rounded stones, and a few pieces of wood. Leave open lanes for swimming.
• Plants: tough stuff that can handle current (Java fern, Anubias, Bolbitis). Floating plants are fine if you do not let them block all surface movement.

Water parameters do not need to be fussy, but stability matters. Neutral-ish pH and moderate hardness are totally workable. What they really react to is dirty water and low oxygen. Keep up with water changes and do not let mulm build up in the flow path.

What to feed them

They are classic omnivores that act like little vacuum cleaners in the water column. Mine take prepared foods readily, but they color up and fill out better when you mix it up.

• Staples: quality flake or small pellets (they do fine with most community foods).
• Protein boosts: frozen daphnia, brine shrimp, bloodworms (not every day, but regularly).
• Small live foods: live daphnia or mosquito larvae if you can source them safely.
• Greens and plant-based: spirulina flakes, blanched spinach bits, or an algae-based wafer broken up for the group.

How they behave and who they get along with

In a group, they are busy but not mean. You will see chasing, especially around feeding time, but it is usually bluff and pecking order stuff. If the group is too small, that energy lands on one fish and you get stressed runts or fin wear.

• Best tankmates: other streamy, non-bullying fish that like similar temps and current (danios, many loaches, some gobies, smaller barbs that are not fin-nippers).
• Bottom buddies: hillstream loaches and similar algae grazers can work well if you keep oxygen high.
• Avoid: slow, long-finned fish (they can get harassed), and super warm-water species that want 78-82F all year.
• Shrimp/snails: adults are usually fine, but tiny shrimp shrimplets are fair game if they can catch them.

Breeding tips

Breeding is doable, but it is not like livebearers where babies just appear. They are egg scatterers. If you want fry, you need to plan for the adults eating eggs (because they will).

• Condition the group: feed heavier for a couple weeks with frozen/live foods.
• Trigger: a cooler period followed by a slight warm-up and a big water change can kick off spawning behavior.
• Spawning setup: fine-leaved plants, spawning mops, or a mesh/egg grate so eggs fall out of reach.
• Collecting eggs: check mops daily and move eggs to a separate small tank with gentle aeration.
• Fry food: infusoria or powdered fry food at first, then baby brine shrimp once they can take it.

Common problems to watch for

Most issues I have seen with Numamutsu are setup-related: not enough oxygen, not enough swimming room, or a group that is too small. Fix the environment and a lot of "mystery stress" clears up.

• Gasping at the surface: usually low oxygen or not enough surface agitation, sometimes a dirty filter slowing flow.
• Clamped fins and hiding: often from ammonia/nitrite spikes, sudden temp swings, or being kept in a tiny group.
• Fin nips and ragged edges: too few fish, cramped tank, or you paired them with other nippy species.
• White spot/ich after new fish: they can catch it like any cyprinid. Quarantine new arrivals and do not swing temps wildly.
• Thin fish despite eating: check for internal parasites and review diet variety. Also watch for bullying at feeding time.