Peacock Eel (Macrognathus siamensis)

What is a Peacock Eel?

The Peacock Eel (Macrognathus siamensis), also called the Peacock Spiny Eel or Siamese Spiny Eel, is a slender, serpentine fish from the family Mastacembelidae — the spiny eels. It is not a true eel at all, but a freshwater teleost that mimics the eel body plan for life in soft-bottomed rivers and flooded fields. The name comes from the row of bold, ringed eye-spots (ocelli) running along the upper body — nature’s bluff to deter predators from above.

Adults reach up to 30 cm (12 in) and are tan to olive-brown, often with a paler cream underside. Their long, laterally compressed bodies taper to a pointed snout ideal for probing sediment. A series of small, sharp spines precede the dorsal fin — handle with care. Kept well, this fish is a genuinely long-lived centrepiece, capable of reaching 8–18 years in captivity with attentive care.

Where do Peacock Eels come from?

Peacock Eels are native to Southeast Asia, specifically the Mekong and Chao Phraya river basins in Thailand and Cambodia. In the wild they inhabit slow-moving or still waters — flooded plains, marshes, irrigation channels, and the weedy margins of rivers. These habitats are typically warm, slightly acidic to neutral, and feature thick layers of soft silt or sand in which the eels burrow to hide from predators and ambush prey.

Understanding this origin is the key to good care. Their biotope offers dim, filtered light under dense vegetation, a loose substrate they can disappear into, and an abundance of invertebrate prey to hunt after dark. Recreating those conditions in the aquarium is not optional — it is the difference between a hidden, stressed specimen and a confident, visible fish.

What size tank does a Peacock Eel need?

A 150 L (40 gal) aquarium is the minimum, and more is always better. At 30 cm (12 in) at full size, the eel needs meaningful horizontal swimming space, and a longer footprint (120 cm / 48 in or more) suits this bottom-oriented species far better than a tall, narrow tank. If you plan to keep community fish alongside, scale up to 200–250 L (52–66 gal) to give everyone room.

The most critical tank element is the substrate: use fine, smooth sand at least 5–7 cm (2–3 in) deep. Peacock Eels burrow headfirst and will spend the majority of daylight completely buried. Gravel prevents burrowing, causes abrasion and skin damage, and produces a chronically stressed animal. Add plenty of cover — driftwood, PVC tubes, caves, and dense planting — so the eel has bolt-holes beyond the sand. Floating plants or tall stem plants help dim the light and encourage more daytime activity.

Lid security is paramount. Peacock Eels are legendary escape artists; they locate any gap and can squeeze through openings that appear impossibly small. Tape over filter inlet holes, cable gaps, and any cover seam.

What water parameters do Peacock Eels need?

• Temperature: 22–28 °C (72–82 °F). A reliable heater is essential.
• pH: 6.5–7.5 — slightly acidic to neutral.
• Hardness: 5–15 dGH, soft to moderately hard.

Filtration should be efficient but produce a gentle current; these fish come from sluggish waters and are not built for strong flow. A sponge filter or a canister with a spray-bar outlet works well. Weekly partial water changes of 20–25% help keep nitrates low — a critical consideration for a fish that spends time in direct contact with the substrate. Never add a Peacock Eel to a tank that has not completed a full nitrogen cycle; the species is sensitive to ammonia and nitrite spikes.

What do Peacock Eels eat?

Peacock Eels are dedicated carnivores with a strong preference for live and wriggling invertebrate prey. In the wild they probe the substrate for worms, insect larvae and small crustaceans. In the aquarium the staple diet should be:

• Live or frozen bloodworms — the most reliably accepted food
• Frozen or live tubifex worms
• Earthworm segments (chopped to an appropriate size)
• Frozen brine shrimp or daphnia as variety

Feed after lights-out when the eel is naturally active and hunting. Most individuals will never accept pellets or flakes; freeze-dried foods are sometimes taken after the fish is well settled, but should be treated as a supplement rather than a staple. Offer food near a burrow entrance or via tongs so it reaches the bottom before other tank-mates intercept it. Feed every second day — overfeeding fouls the substrate where the eel rests.

Are Peacock Eels aggressive — and what fish can live with them?

Peacock Eels are semi-aggressive in a very specific way: they are predatory toward anything small enough to swallow, but largely indifferent to fish too large to eat. Dwarf shrimp, nano fish, small tetras, and any fry will be taken opportunistically at night. Toward fish of comparable or larger size they are generally tolerant, and multiple Peacock Eels can coexist in a sufficiently large tank with ample hiding places.

Good community companions include mid-to-large peaceful barbs, rainbowfish, larger tetras (e.g. Buenos Aires or Congo tetras), gouramis, and similarly-sized bottom fish. Avoid anything nano, delicate, or that shares the bottom space so aggressively it will outcompete the eel for food. Fish that fin-nip or harass a resting eel should also be skipped.

For a full species-by-species breakdown, see Peacock Eel tank mates.

How do you tell male and female Peacock Eels apart?

Sexing Peacock Eels is genuinely difficult outside of breeding condition. As the frontmatter notes, females are noticeably plumper and larger than males when mature; in a group, the rounder-bodied individuals are likely female. Beyond that body shape difference, the sexes look essentially identical — same colouration, same ocelli pattern, same finnage. You cannot reliably sex a solitary specimen. Acquiring a small group of 3–5 individuals and allowing natural pairings to form is the most practical approach for anyone interested in breeding.

How do Peacock Eels breed?

Peacock Eels carry a Very Hard breeding difficulty rating, and this is well earned. Spawning in captivity is rarely achieved and is poorly documented for this specific species. What is known from related Mastacembelidae: conditioning is triggered by a simulated rainy season — a period of slightly cooler, softer water followed by a gradual return to warmer conditions and increased feeding of live prey. Eggs are reportedly scattered among fine-leaved plants or floating vegetation, and neither parent provides significant brood care.

Even reaching the point of conditioning a confirmed pair requires patience; you need a large tank, sexable adults, excellent water quality, and live-food conditioning over weeks. This is a project for experienced keepers only, and most Peacock Eels in the hobby are wild-caught or farm-raised rather than captive-bred through the aquarist’s own effort.

What are common Peacock Eel diseases?

The Peacock Eel’s greatest vulnerability is its skin. As a scaleless fish (it has small embedded scales but lacks the protective armour of most species), it is highly sensitive to external parasites and to salt treatments. Key disease risks include:

• Ich (white spot) — the classic white-grain parasite. Because standard salt-and-heat treatments can harm scaleless fish, rely on heat alone (raising temperature gradually to 29–30 °C / 84–86 °F for 10 days) and consider scaleless-fish-safe medications at reduced doses.
• Skin ulcers and bacterial infections — almost always caused by abrasion from a coarse substrate, poor water quality, or physical injury. The cure is removing the cause: fine sand, pristine water, no sharp decor.
• Internal parasites — wild-caught specimens often arrive with a worm load. A prolonged acclimatisation quarantine (4–6 weeks minimum in a dedicated quarantine tank) is strongly recommended before adding any wild-caught eel to a display tank.
• Stress-related disease — a Peacock Eel that cannot burrow, faces aggressive tank-mates, or is kept in bright, bare conditions will weaken and become disease-prone. Correct husbandry prevents most health problems.

Health note: medication dosing and disease diagnosis are beyond the scope of a care profile. For a sick fish, confirm symptoms against a reputable veterinary or aquatic-health source before medicating — and be especially cautious with scaleless fish, which tolerate many common treatments poorly.

How long do Peacock Eels live?

A well-kept Peacock Eel can live 8–18 years — an exceptional lifespan for a freshwater aquarium fish. This is not a short-term commitment. The upper range of that window is achievable with deep sand, stable water, regular live or frozen feeding, and a tank that is escape-proof and uncrowded. The fish you buy as a juvenile could still be with you well into the following decade. Plan accordingly, and that longevity becomes one of the most rewarding parts of keeping this species.