Gunther's baril

Where they come from

Gunther's baril (Opsarius radiolatus) is a fast-water cyprinid from South and Southeast Asia, where it hangs out in clear rivers and streams with a decent current. Think rocky runs, riffles, and edges where food gets swept past. If you have kept other Opsarius or "barilius"-type fish, the vibe is similar: built to chase and built to live in moving water.

Setting up their tank

Give them room more than anything. These are active, open-water fish that do laps all day, and a cramped tank turns them skittish and nippy. I would treat them like small, streamlined river fish rather than "community tetras".

• Tank size: I would start around 4 ft length for a proper group. Bigger footprint beats extra height every time.
• Flow and oxygen: strong surface movement and good filtration. They look happier when the water is moving and the surface is rippling.
• Layout: open swimming space down the middle, with rocks, rounded river stones, and driftwood off to the sides.
• Substrate: sand or smooth gravel. They are not big diggers, but sharp gravel just adds risk for no gain.
• Plants: optional. If you use plants, stick to tougher stuff (Java fern, Anubias, Bolbitis) tied to wood/rock, or keep plants along the edges so the main lane stays open.
• Lid: a tight-fitting lid. They can and will jump if startled, especially in a new setup.

Water parameters are pretty forgiving as long as you keep it clean and stable. Moderate temps, neutral-ish water, and low-to-moderate hardness have all worked fine for me. What they do not like is stale water, gunked-up filters, and a tank that goes weeks without maintenance.

What to feed them

They are energetic feeders and not picky once settled. Mine took prepared foods fast, but they really color up and fill out when you mix in meaty stuff.

• Staple: a good quality flake or small pellet that stays afloat a bit (they are mid-to-top feeders).
• Frozen: bloodworms, brine shrimp, daphnia, chopped krill or mysis. Rotate so they do not get stuck on one thing.
• Live (if you do it): mosquito larvae, daphnia, brine shrimp. Great for conditioning and getting shy fish eating.
• Extras: occasional insect-based foods are a nice match for how they feed in the wild.

How they behave and who they get along with

Gunther's barils are schooling fish, and you will have a much easier time with a real group. Kept in low numbers they get jumpy, chase more, and sometimes turn fin-nippy because they do not know what else to do with that energy.

• Group size: I would aim for 8-12+ if your tank allows. The pecking order spreads out and they look way more confident.
• Temperament: generally peaceful, but very "busy." Expect chasing within the group and lots of sparring that looks worse than it is.
• Best tankmates: other fast, stream-friendly fish that can handle current and activity (danios, some rainbowfish, many barbs, hillstream loaches, larger Corydoras that like flow).
• Avoid: slow long-finned fish (fancy guppies, bettas, long-fin tetras), timid species that hate commotion, and tiny fish they might harass during feeding.

Breeding tips

They are egg scatterers, and like a lot of river cyprinids, they will eat eggs if given the chance. Spawning can happen in a well-kept group, but raising fry takes a bit of planning.

• Conditioning: heavy feeding with frozen/live foods for a couple weeks, with frequent water changes.
• Spawning setup: a separate tank works best. Use marbles, a mesh floor, or thick clumps of fine-leaved plants so eggs fall out of reach.
• Trigger: a big water change with slightly cooler water can help, plus strong aeration and flow.
• After spawning: pull the adults or move the eggs. Eggs typically hatch in a day or two depending on temperature.
• First foods: infusoria/microworms early, then baby brine shrimp once they are large enough. Fry grow faster with multiple small feedings.

Common problems to watch for

Most issues I have seen with Opsarius-type fish come back to two things: not enough oxygen/flow, or not enough space. They are hardy, but they do not tolerate "stale" tanks the way some calmer community fish do.

• Jumping: usually in the first couple weeks, after a scare, or during chasing. Use a lid and cover gaps around hoses and cables.
• Skinny fish that never fills out: often competition at feeding time. Spread food out, feed more often, and keep them in a bigger group.
• Ich and other stress outbreaks: tends to show up after shipping or big swings. Keep the temp stable, quarantine new fish if you can, and do not let waste build up.
• Fin nipping: most common in undersized tanks or mixed with slow long-finned species.
• Gasping or hanging at the surface: usually low oxygen or not enough surface agitation, especially in warmer water. Add aeration and clean the filter.