Lake Kutubu Rainbowfish (Melanotaenia lacustris) – 4–6cm+ Size Grades

Lake Kutubu Rainbowfish (Melanotaenia lacustris) is a bright blue-green Papua New Guinea rainbowfish for larger, active community aquariums. Adults develop a deep body and metallic turquoise sheen, while a proper shoal creates the constant mid-water movement for which rainbowfish are known.

One quantity supplies one unsexed fish in the selected approximate size grade. Current options are 4–5.5cm and over 6cm. These are dispatch sizes, not adult lengths. Plan a group rather than a single fish.

Lake Kutubu Rainbowfish at a glance

Aquarium size and group planning

Keep at least six Lake Kutubu Rainbowfish together. Eight to ten or more gives a stronger social display where the aquarium has enough length and filtration. These fish are fast open-water swimmers, so a long aquarium is more useful than a tall narrow tank with the same litres.

Use around 200 litres as a practical minimum for a small group, with 240–300 litres or more preferable when keeping a larger shoal or additional community fish. Leave a broad central swimming lane and place plants or wood around the sides and back.

Water and filtration

Keep the aquarium stable at 24–28°C, pH around 7.0–8.5 and moderate-to-hard water. Good oxygenation is important for such active fish. Use mature biological filtration with enough circulation to keep the water fresh without forcing the shoal to fight a narrow powerful jet.

Ammonia and nitrite should remain at zero, while nitrate is controlled with measured feeding, stocking and regular partial water changes. Stable hard water is more important than repeatedly adding products to chase one exact pH.

Feeding

Use a quality flake, small pellet or granule as the prepared staple, with suitable frozen or live foods such as daphnia, brine shrimp, cyclops and bloodworm in rotation. Offer one or two small meals daily and spread food across the surface so every fish can feed.

Lake Kutubu Rainbowfish are enthusiastic eaters, so do not use appetite alone to judge the portion. Persistent leftovers and rising waste mean the amount should be reduced.

Tank mates

Choose peaceful, similarly active community fish that share the same warm, mineral-rich water. Other compatible rainbowfish can work in a suitably large aquarium, provided all groups have enough adult space. Robust bottom dwellers can also work where their own temperature and water requirements match.

Avoid very shy fish that will be overwhelmed at feeding time, persistent fin nippers and aggressive cichlids. Small shrimp and fry may be eaten opportunistically.

Male and female differences

Mature males usually become deeper-bodied and more intensely coloured, while females tend to remain slimmer and less vivid. Small juveniles cannot be sexed reliably, so the current size variants are sold unsexed.

Breeding

Lake Kutubu Rainbowfish are egg scatterers. Condition adults with a varied diet and use fine plants or spawning mops in a dedicated breeding setup. Eggs are laid in small numbers over time, and adults may eat eggs or fry. Remove spawning media or adults when breeding is intentional.

Fry require very small first foods before progressing to baby brine shrimp and fine prepared fry food. Stable clean water and gentle filtration are more important than forcing rapid growth.

Lake Kutubu Rainbowfish questions

How many should I keep?

Keep at least six. A group of eight to ten or more is preferable where the aquarium has enough room.

How large do they grow?

Plan for adults around 10–12cm. The current 4–6cm+ options are approximate dispatch grades.

What tank size is recommended?

About 200 litres is a practical minimum for a small group, with 240–300 litres or more better for a larger shoal or mixed community.

What temperature do they need?

Keep them around 24–28°C in stable, well-oxygenated water.

Do they prefer hard water?

Yes. A practical target is pH 7.0–8.5 and around 10–20 dGH.

Are they aggressive?

No. They are generally peaceful, although males display and the group is very active.

What should I feed them?

Use quality prepared omnivore foods with small frozen or live invertebrate foods for variety.

Can they live with smaller rainbowfish?

Sometimes, but size and feeding speed matter. Very delicate blue-eyes may be outcompeted by a large active Kutubu shoal, so assess the whole aquarium rather than shared family alone.

Can I choose males or females?

No sex is guaranteed by the current size variants.