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Blue Emperor Tetra – Inpaichthys kerri

Blue Emperor Tetra

Tetras are amongst the most popular aquarium fishes. There are probably more than 150 distinct species of tetra from which the aquarist may choose and this includes a large number of visually stunning fishes that are bound to enhance any home aquarium.

Is the Blue Emperor Tetra (also known as the Royal Tetra) a good community fish? The Blue Emperor Tetra should be considered to be an excellent community fish and gets on with most small species. It requires very soft, acidic water so is probably not ideal for novice aquarists. The Blue Emperor Tetra is suitable for novice aquarists and is relatively easy to breed if you get the conditions correct. The Blue Emperor Tetra should not be confused with an Emperor Tetra (Nematobrycon palmeri), an entirely distinct species.

Key Facts

CategoryKey Facts
Common name(s)Blue Emperor Tetra – also known as:
Purple Emperor Tetra
Royal Tetra
Scientific nameInpaichthys kerri
Originate fromAripuanã River in Brazil in South America
Care requiredEasy to care for and hugely popular
TemperamentPlacid, shoaling fish
Colour & FormSilver-blue, torpedo-shaped body with deeper blue band rinning from nose into root of caudal fin under the spine
LifespanUp to 5 years
Adult size1.6 inches
DietOmnivorous – eat aquatic insect larvae in nature
Aquarium size24 inches in length or greater
Compatible withMost other Tetras, Barbs, Danios, Guppies and other livebearers, dwarf cichlids, smaller Gouramis, catfish and loaches
Avoid keeping withLarge and/or aggressive species
BreedingEasy if you put the fish in the right environment.
Water temp75 – 81 Fahrenheit
Water pH5.5 to 7.0
Water hardness (dGH or dH)1 to 8 dGH

Origins of the Blue Emperor Tetra

Tetras, as a “family” of fishes belonging to the biological family Characidae are found in nature in Africa, Central America and South America.

The Blue Emperor Tetra is found in the Aripuanã River in Brazil in South America and, at the time of writing, is the only known species in its genus.

Characteristics of the Blue Emperor Tetra

As you can see, the Blue Emperor Tetra is an impressive little fish. It has a slender body – often described as being “torpedo-shaped”. It will grow to up to around 1.6 inches in the aquarium and lives for two to five years.

Blue Emperor Tetras prefer fairly soft, acidic water with a pH of 5.5 to 7.0 with a temperature range between 75 and 81 degrees Fahrenheit and up to 8 dGH. 

At the time of writing, most Blue Emperor Tetras are wild-caught but I imagine that because there is a vast amount of experience held by commercial breeders this fact will soon become obsolete, as there is certainly a market for this species.

In order to maintain the softness and acidity of the water, it is recommended that the aquarium is filtered using aquatic peat. In addition, floating and naturally decomposing Indian Almond leaves are highly recommended. The floating leaves provide the shaded area that the Blue Emperor Tetra will dwell beneath and the decomposing vegetation provides the tannins that will effectively synthesize the “blackwater” conditions found in the natural habitat of the Blue Emperor Tetra.

The Blue Emperor Tetra is a shoaling fish and should be kept as a group of at least six fish, though a shoal of, say, twenty or more fish is highly recommended. Since this is such a small species, there is every reason to have a large shoal and to include shoals of other small Tetras in the same aquarium.

Having plenty of vegetation to break up lines of sight will also help to protect your Blue Emperor Tetras from predation but it is obviously advised that Blue Emperor Tetras are not kept with predatory species, as they are very small fish and may be seen as a snack.

The body of the Blue Emperor Tetra is typically a silvery blue in colour with a deeper blue, horizontal band running beneath the spine from the mouth, through the eye and along into the root of the caudal fin. 

It is quite easy to distinguish the sex of adult Blue Emperor Tetras because the adipose fin of the male is an orange-red colour whilst that of the female is blue. The fins are otherwise largely clear (hyaline). The Blue Emperor Tetra has a reputation for being something of a fin-nipper but, in my experience, this usually occurs if the shoal is too small or if the aquarium is too barren of plants (as illustrated in the photograph below.

Royal Tetra
Blue Emperor (Royal) Tetras with evidence of fin-nipping. They are in a barren tank.

The Blue Emperor Tetra tends to inhabit the upper area and middle areas of the aquarium (top to bottom). That said, it is a vigorous fish when breeding and will readily traverse the entire aquarium. It tends to feed in the middle area of its environment and, being such a tiny fish will prefer very small food and it loves baby brine shrimp a couple of times each week.

The Blue Emperor Tetra is, by nature, a shoaling fish and it is generally recommended to purchase at least twelve fish – preferably twenty, as their nature is to swim together as a shoal and they will tend to thrive much better as a shoal. Blue Emperor Tetras are excellent community fish and are not ideal for novice aquarists assuming that the more specialized water conditions required are provided and maintained.

Blue Emperor Tetras much prefer a shaded tank, as they can hide from predation and intense light, so consider including floating leaves and/or allowing vegetation to grow so that it floats on the surface of the water to provide shade. They are used to habitats in nature that include decomposing wood and vegetation, which tends to make the water brown (the effects of tannins) and somewhat acidic.

It is generally advised that the minimum tank size for Blue Emperor Tetras should be one of at least 24 inches in length or more due to the shoaling nature of the species, which will enable a small shoal to move around freely. There is a predominance of so-called “Nano tanks” available but, being old-fashioned, I prefer my fishes live in an environment which, at least, attempts to mimic nature, rather than living in what I would liken to a prison cell. The tank should be well-planted but with clear areas where the fish can swim freely. The water should have gentle movement.

Blue Emperor Tetras are difficult to sex until they are mature, where the female has a slightly fuller body when she is carrying eggs (gravid) and a longer caudal fin than the male and the male is likely to take on richer colouring with more green pigmentation. 

The general rule for Tetras is that by keeping six or more of the same species in an aquarium they will be fully aware of which is male and which is female and they will act accordingly when the time comes for breeding.

Most (but not all) Tetras have an additional fin which identifies them as being Tetras and the Blue Emperor Tetra is no exception. If you look between the dorsal fin and the caudal (tail) fin of the Blue Emperor Tetra you will observe a tiny, additional fin, known as the adipose fin. The purpose of this fin is not fully understood but, if it is present on a freshwater tropical fish then you can be pretty certain that the fish is a Tetra.

Tetra comes from the Greek word “Tetragonopterus” which means square-finned and appears to relate to the four fins on vertical plane of the fish (dorsal, adipose, caudal and anal fins) which span the central line of the fish (when viewed from above or below, front or rear) and are not present as a pair (e.g. the pectoral or pelvic fins).

Blue Emperor Tetra – Video

How do Blue Emperor Tetras breed?

Tetras, in general, will scatter eggs by laying them over fine plants such as Cabomba, Fontanalis or Java Moss.

Blue Emperor Tetras, like most species, are noted to leap above the water surface during breeding and in general, so it is advised that the tank should be covered to mitigate the risk of losing fish.

As the female Blue Emperor Tetra becomes ripe with eggs, the difference between the sexes may become more evident, as the body expands because it is carrying eggs. If you plan to attempt to breed Blue Emperor Tetras then it is recommended that you have a breeding tank prepared. Such a tank can be empty but you may wish to include a mesh or grid under which the fertilized eggs will fall and/or a sterilized breeding mop.

That said, Blue Emperor Tetras may breed in a community tank and, if the tank is well planted, it is possible that at least a few fry may survive to reach adulthood. Having a good covering of Willow Moss on the floor of the aquarium seems to provide a safe haven for fry, which can help them to survive predation in a community or single-species tank.

Some say that a novice may find it difficult to breed Blue Emperor Tetras but, in my experience, by understanding the conditions that are ideal for breeding, most species will breed quite readily, as it is natural for them so to do and, in general, nature will find a way. Ensuring that the pH is close to 6.0, the dGH is around 4 to 6 and the water temperature is gradually raised to around 80 degrees Fahrenheit (over several days), together with feeding plenty of tiny, live food (such as baby brine shrimp) are all likely to contribute to successful breeding.

Not raising the water temperature will not prevent the Blue Emperor Tetra from doing what comes naturally, it is merely a natural sign in nature that it is time for them to breed.

The female will swim more vigorously around the tank and, if you include two males then they will encourage her to lay her eggs by bumping into her. She will lay her slightly adhesive eggs which will immediately be fertilized by the male(s) and will either stick to plants or spawning mop or will fall to the bottom of the tank (preferably through the mesh or trap). Once spawning is complete, remove the adults, as they are likely to consume the eggs, given the chance, and take no further parental responsibility.

With this in mind, it is obviously necessary to prepare two tanks – one in which the breeding takes place, the other with identical water conditions and temperature into which the adults are placed after spawning is complete and before they are returned to the community tank, having slowly reduced the temperature to that of the community tank over, say, one week.

Spawning usually takes place in the early morning and, in nature, it is the rising of the sun which triggers breeding amongst most Tetras. 

Breeding tank for Blue Emperor Tetras

You should prepare a tank of around 5 gallons in size with mature, soft, acidic water. The water should have a low level of light.

You may wish to introduce baby brine shrimp, mosquito larvae or bloodworm as an inducement to reproduction.

The female will swim amongst the plants, laying around 100 to 150 eggs during a spawning. The male(s) will swim alongside or behind her and fertilize the eggs as they are laid. Once the female has laid all her eggs the adults should be removed from the breeding tank.

Keep the lights off and the tank dark because Tetra eggs and fry are particularly sensitive to the light.

The eggs will hatch typically in a day or two depending on tank temperature and conditions and the fry will become free-swimming after around three to four days after hatching. Keep the tank unlit for the first week or so then gradually increase the lighting. Bear in mind that the eggs and fry of a fish as small as the Blue Emperor Tetra will be tiny indeed so you may need to use a magnifier “app” on your smartphone to see anything at all.

The newly hatched fry will feed firstly on their yolk sac but, once free-swimming, can be fed infusoria (particularly rotifers) and will also thrive on egg yolk during the first two to four weeks. It is worth mentioning that immediately after hatching, fry seem quite vigorous but will soon go into a resting state before they become free-swimming so please don’t mistake this initial stage as being free-swimming.

After around four days or so add baby brine shrimp. Once the fry are sufficient in size not to be treated as a snack then they can be introduced into the community tank where they will join the existing shoal. Before moving the adolescent fish into the community tank ensure that you have balanced the water temperatures to mitigate the risk of White Spot or other diseases being triggered.

Should your Blue Emperor Tetras have a special diet for breeding?

Adult Blue Emperor Tetras don’t need any particular inducement to breed. That said, it has been suggested that adding baby brine shrimp, mosquito larva may encourage them, presumably because the addition of a new food may “fool” the fish into thinking that it is breeding time. From my own experience, I would always recommend keeping all of your fish in the best possible condition at all times, as this is good for the wellbeing of your fish.

Featured image courtesy: Wikimedia Creative Commons

Mike Wheeler

I started keeping freshwater tropical fish in 1972 and it has been something of a passion ever since. In this website, my aim is to build up an everyman's guide to help the everyday aquarist get the best from this inspiring and entertaining hobby.

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