How to darken Serrasalmus rhombeus in the home aquarium

By Frank Magallanes and contribution by Edouard Paiva

Photos seen here are the property of Edouard Paiva. OPEFE use only. Permission to use outside of OPEFE requires photographer permission.

The problem of rhombeus nomenclature and description is a big one. There are several forms widely distributed. The form living in the Guiana's are slender, while forms living in the Orinoco, Amazonas and other South American rivers are deeper and very dark. The only thing common is that they have a red eye. Scientists believe that S. rhombeus is a complex of species. Both Dr. W.L. Fink and Dr. Antonio Machado-Allison are still working with the Brazilian species. According to Nakayama et al (unpubl) fishes from Catalão Lake (a mixed water ecotone) which is formed by water from the Solimões River (white waters) and the Negro River (acid and dark waters), this environment caused  some changes in chromomsomes in this species. This melting together (or fusion) of the centromere (=the centromere consists of fibres and holds together two to create a chromosome) by the eggs released by these species may have been caused by interbreeding. In previous discussions with a field researcher (1995) I discussed this topic of Serrasalmus species (in particular S. spilopleura) interbreeding and new populations forming after being cutoff from main rivers. At that time he thought it was unlikely. This new DNA information is changing this perception.


There seem to be several complex forms which vary in coloration and spotting. The distinctive red eye is present on all of them after some reach 5 inches or larger in sizes, some could be smaller in size. Their body color varies from silver to black with spotting variable at all ages. The anal fin has a reddish tint, later during ontogeny progressing to hyaline. A dark black margin on the anal fin is present on young specimens, later fading in intensity on adults. I have seen S. rhombeus belly colors also go through many ontogeny changes, from red, pink, yellow, gold to white. With maturity, the belly color may become black or white, with the upper body dark gray giving it a marbled appearance. With Peruvian S. rhombeus, the head may be bright yellow.


The white color often attributed to the species S. rhombeus was based on a juvenile form, however, water conditions can produce this effect on adults. The South American Indians are the folks generally responsible for providing common names and Piranha Branca (white piranha) is one of them. As Petry mentioned above it is only natural the darker forms of rhombeus would be called Piranha negra (black piranha) by natives. The common name itself has no scientific value and the Indians use this name on quite a few number of dark piranhas without discriminating the species it is. Presently, some species are being offered as Araguaia or Xingu "rhombeus." I have been afforded the opportunity to examine some of these species and most appear to be Serrasalmus rhombeus. There might be some variation to spotting and elongation of the body, but the majority seem to be this species. No systematist has yet to publish otherwise. 


Recommend keeping as solitary species in home aquarium. Large specimens like the one above should be kept in dimly lit aquariums. These larger one's inhabit deep waters where sunlight is less likely to penetrate. They are river fish and require strong current. I further suggest keeping them in a deep square tank than a long tank. Dimensions should allow sufficient turn around for the fish so that it will avoid bent tail syndrome, a feature seen on some fishes where the bottom or top of the caudal fin is crooked. This sometimes occurs from a fish being kept in a container that is to small for the fishes requirements. A power head is a must for flow and to keep the fish healthy! Water temperature should range from 76-82F. Soft, slightly acid preferably between pH 6.8 to 7.4. In a topic further below, I have included additional information on water chemistry reader's should consider.

S. rhombeus from white water looks washed out, almost completely white. This white appearance gives S. rhombeus a strange appearance. Clear and black waters have fish with deeper color. The darkest S. rhombeus ever seen were in the upper Uraricuera, which is clear water. Nevertheless, the Rio Negro fish are quite dark as well. Somehow black water creates great color contrast. Tannins are added in the home aquarium to enhance the colors, also tannins also help facilitate breeding in most piranhas.

So how does one provide an aquarium that will best enhance the features of S. rhombeus?

With that we must travel to Venezuela and my friend Edouard Paiva. Edouard has kept species of piranhas for some time and has fished for them in Venezuela rivers. In conversation with Edouard via email, we came up with a solution on how to keep his S. rhombeus in dark color and health.

Below are some results in Edouard own words:

The first picture below was taken 2 days before I used the Tetra Blackwater Extract and Vita Chem.

S. rhombeus before blackwater treatment and change of gravel. Photo by Edouard Paiva

 I ordered them to Amazon.com because you can not find those products here in Caracas, Venezuela.

I had to wait 3 weeks before I receive and used the products. I saw good results the next day and after 2 days it was darker than before. When you saw the good results of the tonic you recommended to add the much darker gravel to optimize the results and make the rhombeus darker, I ordered it and had to wait 2 more weeks and the results after the darker gravel was added were great after 2 days it gained the great dark color you see in the 2 pictures.

S. rhombeus, two days after treatment. Photo by Edouard Paiva. OPEFE USE ONLY

The results were very fast but it lasted a lot more time because you do not find the products and the gravel here in Caracas, Venezuela.

S. rhombeus, close up. Photo by Edouard Paiva. OPEFE USE ONLY

Results may vary depending on several factors, including water chemistry, stress, gravel, background and lighting.


The OPEFE web site and its contents; is disclaimed for purposes of Zoological Nomenclature in accordance with the International Code of Zoological Nomenclature, Fourth Edition, Article 8.3 and 8.4. No new names or nomenclature changes are available from statements at this web site.

CopyrightŠ 1994-2012 Oregon Piranha Exotic Fish Exhibit (The OPEFE fish exhibit is permanently CLOSED as of 2000) Sutherlin, Oregon. Information posted on this web site is archival data on fish scientific classifications and other information. DISCLAIMER: The copyrighted material may not be used for any purpose other than private study, scholarship or research. Cited information requires credit and this link www.opefe.com. All rights reserved. All images shown  (unless otherwise noted) is property of OPEFE. 

UPDATE: 12/10/2015