Siamese algae eaters, black beard algae, and what they do to other algae

Which fish is the actual Siamese algae eater?

This is where most fishkeepers go wrong before they even get home from the shop. The name "Siamese algae eater" gets applied to several completely different fish, and the one you actually want - a true Crossocheilus - is often not the one in the tank.

The fish most reliably identified as true Siamese algae eaters belong to the genus Crossocheilus, most commonly Crossocheilus oblongus and closely related species. Look for:

• A continuous dark lateral stripe running from the tip of the snout to the base of the tail - and crucially, it extends onto the tail fin lobes, which most lookalikes do not have.
• A slightly ragged or irregular upper edge to the stripe, not a clean geometric line.
• A streamlined, torpedo-shaped body with a small, fleshy barbel at the corner of the mouth.
• A semi-transparent lower lobe to the tail and generally muted, non-iridescent colouration.

Juveniles show bolder contrast - nearly black-and-white - that softens into the adult pattern around 5-7 cm.

The flying fox (Epalzeorhynchus kalopterus) is another common mix-up. It has a cleaner, more defined black stripe and a vivid gold stripe above it. Adults become territorial and eat far less algae than a true SAE. The Siamese flying fox (Crossocheilus siamensis) is closer to the real thing and sometimes what you actually get - it does eat BBA - but even here, species sold under that name are inconsistently identified in the trade.

When buying, bring a reference photo from a reputable source and compare the tail stripe specifically. If the stripe stops at the body and does not bleed into the caudal fin, walk away.

Black beard algae: what SAEs actually do

Black beard algae - BBA, Audouinella sp., sometimes called brush algae - is a red algae despite its near-black colour. It anchors to wood, slow-flow corners, plant leaves, and filter intake edges, and it is notoriously resistant because it grows where conditions are already suboptimal. Once it matures into dense, wiry tufts, it is tough.

True SAEs are genuinely useful against BBA, but with a specific and limited brief:

• Young, soft growth: SAEs actively graze the early, fuzzy stage of BBA before it hardens. This can meaningfully slow an outbreak if you catch it early.
• Post-treatment maintenance: After you have manually removed mature BBA and corrected your CO₂, flow, and nutrients, SAEs help prevent re-establishment by picking off new growth before it gets a foothold.
• Established mats: They will nibble around the edges but will not strip a thick BBA carpet. Do not expect them to fix an existing problem.

For a full breakdown of what actually drives BBA and how to clear it, see our aquarium water parameters guide - specifically the sections on CO₂, phosphate, and circulation balance.

What else do they eat?

True SAEs are opportunistic grazers, not specialists. Their diet in a tank includes:

• Biofilm and microalgae coating glass, rocks, and wood - they are consistently good at this.
• Soft filamentous algae - green thread algae, hair algae in early stages, and similar fine-textured growth.
• BBA - as described above, with caveats.
• Prepared food - they will eat sinking wafers, algae tabs, and most foods that reach the bottom. They compete actively at feeding time.

They do not reliably eat staghorn algae, green spot algae on glass, or cyanobacteria (which is not a true algae anyway). They also ignore green water. If your tank has a specific algae problem, check which species you are dealing with before buying fish on the assumption they will fix it.

Tank requirements

Size and flow

SAEs are native to fast-moving Southeast Asian rivers - Thailand, the Malay Peninsula - and they need good water movement. A sluggish, heavily planted tank with a gentle internal filter is not a great match. They do best with:

• Tank size: 120 litres (30 US gallons) absolute minimum for a small group of adults. Bigger is better - these fish cover ground quickly.
• Flow: Moderate to strong. A turnover of 8-10× the tank volume per hour suits them. They will actively swim against a current.
• Length over height: A long, shallow tank serves them better than a tall display tank. They are horizontal swimmers.

Water parameters

SAEs are reasonably adaptable, but they come from soft, slightly acidic water in the wild. Target:

• Temperature: 24-26 °C (75-79 °F). They tolerate 22-28 °C but are less active at the extremes.
• pH: 6.5-7.5. They handle a slightly wider range but do not push it.
• Hardness: Soft to moderately hard - 2-15 dGH. Avoid very hard, alkaline setups if you can.
• Ammonia/nitrite: Zero, as with any fish. Check your nitrogen cycle is complete before adding them.

Social behaviour and stocking

SAEs are shoaling fish and do better in groups of four or more. Kept alone or in pairs, they can become skittish or mildly aggressive toward similarly shaped tank mates. In a group, they are active, peaceful mid-to-bottom dwellers that work well in most community setups.

They do not play well long-term with very slow, flat-bodied fish if kept hungry - like the Chinese algae eater, a poorly-fed SAE may investigate the slime coat of a discus or a large fancy goldfish. Keep them well fed and this is rarely an issue.

Feeding

SAEs will eat algae willingly when young, but they also need regular feeding with sinking foods - algae wafers, blanched vegetables, and quality sinking pellets. An underfed SAE becomes a problem fish: more aggressive, more likely to harass tank mates, and - ironically - more destructive toward plants as it starts sampling leaves. Feed once daily minimum, and do not rely on tank algae as their primary food source.

Age and the algae-eating drop-off

This is what most guides skip. SAEs are enthusiastic algae grazers at 3-8 cm. As they approach adulthood - around 10-12 cm - their interest in algae decreases noticeably. Adult SAEs spend more time competing for prepared food and less time methodically grazing surfaces.

This does not make them useless - they still browse and they still help with BBA maintenance - but if you are buying adult-sized fish expecting the same grazing output as juveniles, you will be disappointed. The sweet spot for algae control is the 4-9 cm range. Plan your tank accordingly and do not expect the same fish to do the same job at five years old that it did at one.

Practical BBA management: the honest sequence

If you have a BBA problem and are considering SAEs, here is the actual order of operations:

1. Diagnose the cause. BBA almost always comes down to CO₂ fluctuation, low or inconsistent CO₂ in planted tanks, slow flow creating dead spots, or nutrients (usually phosphate) running high. Test your water. See aquarium water parameters for what to check.
2. Correct the underlying issue. Stable CO₂ injection, improved circulation, and adjusted nutrient dosing. Without this, nothing else holds.
3. Manually remove mature growth. Use a toothbrush, scissors, or spot-treat dense mats with diluted hydrogen peroxide (3% solution, applied directly with a syringe with flow off for 5 minutes, then water change). Remove what you can physically.
4. Add SAEs. Now that conditions are improving and mature BBA is reduced, a group of young SAEs will graze regrowth and help hold the line.
5. Maintain parameters consistently. Use a tool like App-aquatic to log your CO₂, nitrate, and phosphate readings over time - pattern recognition matters more than any single test result.

Skipping straight to step four without doing the others is why so many people report that "SAEs did nothing for my BBA."