Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Nannostomus marginatus

Dwarf pencilfish

Dwarf pencilfish, Nannostomus marginatusDwarf pencilfish, Nannostomus marginatus
Photos by Andy Rapson


Latin, nannus = small.
Greek, stoma = mouth

Refers to the small mouth of this genus.

General Notes:

Wild caught N. marginatus, require soft acidic water with very low dissolved solids. commercially bred fish will be a little more forgiving than wild caught specimens but both will fail to thrive in harder alkaline water. Although these fish do much better when kept in a group, they won't normally swim around as a group but will take up small territories and spar with each other near the territory border. Using subdued lighting and a dark substrate will show these fish off at their vibrant best, bright lighting and pale substrates will cause the fishes colours to fade. N. marginatus is a sensitive species and should not be added to any aquarium which isn't fully mature because even a trace of ammonia or nitrite may well prove fatal.


Nannostomus marginatus, is a micro predator which preys on small aquatic inverts in the wild. In captivity care must be taken to provide food with the right particle size for their small mouths. Small live or frozen food such as daphnia, cyclops and small brine shrimps along with a good quality flake or granular food will provide them with all that they need.


Very peaceful with other community fish which require the same environment and of a similar size. N. marginatus is a shoaling species and must be kept in a group of at least five individuals. Don't keep them with very lively robust species which may out compete them for food.


Males are more slender than females and are more intensely coloured.


N. marginatus are difficult to breed but if kept in a planted tank very occasionally fry will appear with out any action being taken by the fish keeper. Under such circumstances only a very small minority of fry (if any) will survive.

A separate breeding tank should be set up with very soft water and allowed to stand for two or three days. Use a black water additive to gently acidify the water and to slightly stain it. The aquarium should have a mature air powered sponge filter and some form of spawning mop - artificial or fine leaved plants will do. The tank should be placed in a permanently shady area and no lighting should be used because the eggs and young fry are light sensitive.

If you have a group of these fish which have been spawning in a community aquaria, Place them in a separate tank with the sexes kept apart and condition them well for about a week. Then transfer both sexes of the fish to the breeding aquaria. Don't use too many adults because this will increase the risk of egg predation. Leave the adults in the breeding tank for a couple of days, if spawning is going to take place it will have done by day two.

Don't feed the adults in the breeding tank.

The fry will be free swimming after about six days when they should be fed initially using infusoria, rotifers, hard boiled egg yolk mixed with yeast or a liquid fry food. Then when they are big enough use freshly hatched brine shrimps and micro worms.

If all goes well in eight weeks the fry should be about 1cm (0.5in) and beginning to colour up.

Wild status

The IUCN Red List of endangered species has not evaluated Nannostomus marginatus. Most specimens offered for sale have been commercially bred.

Additional information

N. marginatus make great dither fish for various acid loving species like Discus, Apistogramma, Dicrossus and other similar fish.

Information at a glance

pH: 5.8 - 6.5
dGH: 3 - 10
Temperature: 22 - 26°C (72 - 78°F)
Lighting: Subdued
Diet: Omnivore
Size: 3.5cm (1.5in)
Min tank size: 100 litres
Difficulty level: Easy
Aquarium type: Community
Swimming level: Middle

Distribution and habitat

distribution map for Dwarf pencilfish, Nannostomus marginatus

Origin: Lower to middle Amazon river - Colombia, Guyana, Peru, Suriname, Venezuela

Habitat: Heavily vegetated, slow moving streams, rivers and swamps.


Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Characiformes
Family: Lebiasinidae
Genus: Nannostomus
Species: N. marginatus, Eigenmann, 1909

Other common names:
Striped pencilfish

Nannostomus marginatus picturatus