Name: Serrasalmus rhombeus
Origin: Amazon Basin
Probably the most common piranha from the Serrasalmus group, the S. rhombeus is one of the more fierce looking varieties available. With powerful jaws and lightning quick movements, this piranha can cause serious injury and often death in all tankmates and should always be kept solitary. Even tankmates of the same species will not work and this piranha has in fact only been bred in captivity twice, both times the batch sustained heavy losses to cannibalism. Being that breeding in captivity is all but impossible, all specimens seen for sale have been imported from the wild. These fish generally grow at a rate of less than 3 cm per year, and rarely exceed 30 cm if grown in captivity. Specimens of 35 cm or more are usually caught this way as few captive rhoms will reach such sizes.
Various geographical variants of rhombeus do exist, with slightly different shape, coloration or amount of reflective scales making up the major differences between the types commonly offered. An important point of note is that most pet stores label all their Serrasalmus species as black piranha, when in fact they may be other species such as S. sanchezi, S. compressus or even S. spilopluera and S. brandtii. Since all of these fish should be kept solitary, and they have similar dietary requirements, most hobbyists never know the difference.
Though the common misconception is that piranhas need to be fed live food. They are much healthier on a diet of more nutritious foods, such as frozen smelt, catfish, white fish filet or uncooked shrimp. Some rhombeus refuse to eat anything but live food at first, but it is important to remember that this fish will not starve itself to death if frozen food is available. They can go over 3 months without eating and will not starve to death, so the important thing in weening them off feeders is to have patience and keep offering prepared food every few days until it does eventually accept it. Mine took three weeks of not eating before he would finally accept shrimp, today I have no problems getting him to eat.
As with all piranhas, the rhombeus can be a nervous and skittish fish at times, or it may be an outgoing active pet. They generally take a long time to feel comfortable in new surroundings and around people, and it may take as long as a few months before your fish starts showing it's true personality. It is not uncommon for them to refuse food during the acclimation process to a new tank, and a good practice is to leave some food in the tank with the lights off and remove the uneaten food in the morning or after an hour or so.
Tank sizes will vary depending on the size of the fish. For life, they will need to be housed in a 450 liter or greater when they reach maximum size. However, because of their slow growth rate, they can live for many years in a 150 liter or 200 liter tank. Common sense should tell you when you need to upgrade to a larger tank, however a good guideline is that when they get past 20 cm you should start looking for a bigger tank for them.
The one rhom that I keep was bought at about 15 cm and he's uncharacteristically friendly and outgoing...if you could call biting at me through the glass being "friendly" that is. He takes an interest in everything and everyone outside of the tank, and usually comes up to the front to get a closer look at anyone passing by or even looking into his tank. I'm keeping him in a 150 liter breeder which I've heavily planted and I've noticed a definite preference for the open areas of the tank over those with obstructions, like plants or driftwood. Accordingly, the tank has been decorated to give him lots of swimming space keeping things like driftwood and tall plants along the glass walls and keeping anything in the center of the tank no taller than 8 cm. Most of the day he spends casually swimming in the current from my powerhead and, while he definitely doesn't use more than a third of the tank very often, the occasions that he does dart to the other end of the tank make me glad he does have the room if he needs it. I'll probably get him a nice 450 liter in a few years, which seems like a lot for a single fish, but he's grown on me a lot in the time I've had him. Some people would argue a 30 cm fish is fine in a 300 liter, but the speed with which they swim wouldn't have me feeling comfortable putting him in anything else.
I have been keeping piranhas for around 25 years. In that time I have only really kept Serrasalmus rhombeus and S. gibbus (Blacks and Golds). Some advice to new owners is:
1) Although these species do not require any extensive care, a commitment is still required. If you have a 100 liter tank and see a 10-12 cm black rhom or gold you will probably need to upgrade your tank to a minimum of 200 liters with at least 400 GPH of filtration. I've raised mine in 245, 265 and 340 liter tanks. Highly consider a 55 liter feeder tank with 100 GPH of filtration. There is nothing worse for your ammonia level than the introduction of a few feeders into the tank. Not only when it rises with the initial introduction of eight to ten feeders, but when your black rhom eats two and kills the rest your water quality has been compromised. I purchase eight medium size feeders, ones that can be devoured in two bites and then after eight days I do a 40% water change to the feeder tank and let the new feeders sit in the tank for two to three days.Besides healthy feeders, only feed the piranha RAW meats like chicken and small portions of lean beef. Farm raised earthworms are a nice treat, as well as seafood. Never hot dogs, pork, cooked meats or insects found in your garden. The cooked meats produce grease that give the species problems and outdoor worms could contain those lovely garden chemicals used extensively.
2) If you are looking for a species to entertain guests while having a party, please do not select a rhom or gibbus - get an Oscar. In all my years of piranha keeping, rarely did any of my single species immediately devour feeders in front of me, let alone 8 people standing around the tank. Piranhas are purchased with this entertainment in mind, but beware it could be some time before the piranha eats in front of you. I've seen people purchase piranhas with this in mind only to find themselves returning the fish after a month because it is a boring species. BE PATIENT, let the Rhom adjust to its new environment. Keep this species in tank in a corner alone - they spook easy. If your piranha spooks and inflicts a gash upon himself, turn the light out and wait about ten minutes, then carefully turn the light on to assess the damage. Normally medication is not required as these fish have unbelievable healing powers. This is also the reason that I only keep Amazon swords in the tank and not rocks and driftwood and I use a background that covers three sides of the tank.
3) Water changes are important, as these predators create a high level of ammonia just by their feeding regimen. I currently have a 23 cm rhom in a 245 L tank (soon to be move to a 340) and I do 75 L water change every 10-14 days. If your water quality begins to suffer, the first sign is usually that the fish's eye begins to cloud. This is a sign to change water and consider a carbon change in the filter.
4) If you need to medicate a piranha, never use a wide spectrum medicine (i.e., medicate the entire tank). I will not feed mine for three days, after which I will cut a pocket in a piece of raw chicken and then put a small amount of medicine in the food. I found this to be far and away more effective than using one teaspoon for every 10 L of medicine and you don't have to remove your carbon cartridge.
Thanks for your time to read my experiences. Just be patient with your piranhas and they will reward you with years of enjoyment.
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