Iberian arch-mouthed nase

Where they come from

The Iberian arch-mouthed nase is a little river cyprinid from the Iberian Peninsula, mostly Portugal and western Spain. Think clear, flowing streams with seasonal swings: cooler, oxygen-rich water in winter and spring, then lower flows and warmer temps in summer.

That seasonal rhythm shows up in how they eat, color up, and (if you ever try it) spawn. They are not a "pond goldfish" type of fish - they are built for current.

Setting up their tank

If you treat this like a river fish, your success rate goes way up. Big footprint matters more than height. I would rather run a long 4-foot tank than a tall cube for these.

• Tank size: I would start at 40 breeder/75L long as an absolute minimum for a small group, but 4-foot (55-75 gal) is where it gets comfortable.
• Group size: 6+ if you can. In small numbers they get skittish and you see more fin nips.
• Flow and oxygen: strong filter turnover plus a powerhead or river manifold. They like to sit in the current and pick at surfaces.
• Substrate: sand and smooth pea gravel. They root and graze, and sharp gravel just beats up mouths and bellies.
• Hardscape: rounded stones, cobbles, and some driftwood. Leave open lanes for swimming and a few "breaks" from the current behind rocks.
• Plants: optional. If you use them, go for tough stuff (Anubias, Bolbitis, Java fern) tied to rock/wood. In high flow, delicate stems get shredded or uprooted.

Water numbers: neutral to slightly alkaline is usually fine, but stability matters more than chasing a perfect pH. Keep nitrate low, do real water changes, and do not let mulm build up in dead spots. Temperature-wise, they do best on the cool side for a "tropical" fish - I like low 60s to low 70s F (around 17-23 C), with a winter cool-down if you are thinking about breeding.

What to feed them

In my tanks they behave like constant grazers. They will take pellets, but they really look their best when they can pick and browse all day.

• Staple: quality sinking pellets or wafers (not just one brand forever). I rotate a couple.
• Greens: blanched zucchini, spinach, green beans, and algae wafers. Clip it down so it stays in the flow and they can work at it.
• Protein: frozen foods like bloodworms, daphnia, cyclops, and brine shrimp a few times a week. Not huge amounts - just enough to round them out.
• Natural grazing: let some rocks and wood grow a light film algae/biofilm. They will spend hours on it.

How they behave and who they get along with

They are active, alert, and kind of "busy" fish. Not aggressive in the cichlid sense, but they do have a pecking order and they will bicker, especially at feeding time.

Best tankmates are other cool-water, current-loving fish that can handle a fast tank and do not get bullied off food. Think small to medium cyprinids from similar setups, or hardy loaches that like flow.

• Good matches: other rheophilic minnows/barbels (similar size), hillstream loaches, some smaller loaches that like current, tough cool-water species.
• Be cautious with: long-finned or slow fish (they get stressed and outcompeted), very tiny fish (can be chased), and anything that needs warm, still water.
• Avoid: fish that require high temps (mid/upper 70s F and up) or calm planted-tank conditions. You will end up compromising one side or the other.

Breeding tips

Breeding is possible, but it is not a casual community-tank thing. In nature they cue off seasons and flow changes, and you basically have to mimic that.

• Conditioning: several weeks of heavier feeding (greens plus frozen) while keeping water very clean.
• Season cue: a winter cool period (cooler temps and shorter photoperiod), then a gradual warm-up and big fresh water changes.
• Spawning setup: shallow-ish tank with strong flow over rounded gravel/cobbles. Some people use a "river" manifold so the eggs do not sit in muck.
• Egg safety: adults will eat eggs. If you see spawning activity, either pull adults or give the eggs a way to fall into gaps where adults cannot reach.

Common problems to watch for

Most issues with this species trace back to the tank feeling like a stagnant aquarium instead of a stream.

• Low oxygen: gasping near the surface, hanging by the filter outlet, acting "lazy". Fix flow and surface agitation first.
• Chronic nitrate and detritus buildup: fish slowly lose weight and color even though they eat. Vacuum dead spots and increase water changes.
• Mouth and barbel wear (if kept on sharp gravel): you will see redness or fraying. Switch to sand/smooth gravel.
• Internal parasites/wild-caught issues: skinny fish that eat but do not fill out. Quarantine and consider targeted deworming if you know what you are doing.
• Outcompeted tankmates: slower fish starve while the nase bulldoze food. Spread food along the current line and use sinking foods in multiple spots.