The following is a thread that I started at another site that may contain some useful information regarding the topic of growing Serrasalmus Rhombeus in captivity. Please feel free to post your views and experience on this subject, as this is how we all have the opportunity to learn from one another.
Recently I had the opportunity to talk with Oliver Lucanus about the availability of different rhom variants. I mentioned that I was looking for a particular rhom from the Orinoco river system, and needed it to be at least 13" in length. Oliver stated that he did not understand why so many people are willing to pay huge amounts of money for large rhoms, when they can be easily and quickly grown in the home aquaria. I then expressed to him that the common knowledge about captive rhom growth that I had heard over the years was that growing a rhom to a size much larger than 12" in captivity is almost unheard of. Oliver said that this line of thinking was nonsense, as he has on many occasions seen rhoms in Asia that were raised from 5 + 6" fish, that grew to sizes larger than 16" in captivity. When I asked him if they were doing things differently than the norm, this was his response.
The first thing that Oliver said that was lacking in most of our rhom tanks is a sufficient amount of current. He stated that rhoms are typically found in much faster water than pygos, and the Asians have learned to reproduce this flow in their home aquariums. This made sense to me, since they would expend a lot more calories striving to hold position against the current. They would therefore need to consume a larger quantity of food, which would in turn promote better growth.
The second thing that Oliver mentioned to assist in growing out a rhom was to have an automatic drip system to keep the nitrates down to a minnimum. Apparently these systems are much more common in Asia, and he believes that this too plays a huge role in helping the rhombeus to grow large in captivity. I am very familiar with the automatic drip system, since I have used it to grow pygos in my own tanks, so this too was a method that I was in complete agreement with.
Although it wasn't mentioned in our conversation, a quality and varied diet is also something I believe is crucial in growing a large rhombeus, or any piranha for that matter. It has always been my belief that whole fish (with heads and guts in tact) should make up the bulk of this varied diet, as this is what our adult flesh eating rhoms consume most of the time in their natural habitat. I also like to regularly rotate shrimp into the mix, and periodically will throw in some earthworms or catfish fillets as a change of pace. On very rare (due to the risk of disease) occasions, because I still greatly enjoy watching piranhas in the act of predation, I will feed live fish stuffed recently fed with a high quality flake food or pellet.
I am trying out these methods with a new rhom that I have recently acquired, and will keep everybody posted on the results. He is currently between 11-12" long. Of course, I still have my theory that one of the reasons our captive rhoms do not grow as well as pygos is because they are not forced to compete for food in the home aquarium. In most cases the captive rhom is kept solo, whereas our pygos shoals seem to sometimes eat just to keep their tankmates from getting any food. I am hoping that the added current will counter this pickyness that many of our rhoms often times exhibit, but time will tell. UPDATE: I have now had the above mentioned rhom for around six months now. He has grown to about 12", and I think he was closer to 11" or so when I got him. Although that is not all that significant when it comes to growth even for a rhom, keep in mind that the first few months I was fasting him periodically, to train him to eat the foods that I wanted to feed him to promote good health and growth. Due to the very stubborn nature that many of our captive rhoms seem to possess, he ate very sparsely at first. I expect that he will begin to grow at a much more accelerated rate, since he is now readily and aggressively eating all the foods that I want him to.
Anything anyone can tell me to help my GD rhom to grow is very much appreciated,i respect this mans attempt at helping my rhom attain more growth.I too believed in the 50% water change weekly,but since i started doing 2 - 10% weekly water changes plus a power head as well as adding 12 tetra black skirts my rhoms appettite has almost doubled,he used to eat everyday (4 - 5 bites of shrimp) but now he devours a whole shrimp at feeding time.My rhom was 5.5" when i put him in my 125 gal tank 2 weeks ago,and i swear he has grown at least a .5" since.I truely believe between the power head and the tetras(maybe the rhom sees these fish as competion for food) has helped my fish grow somewhat.Great thread and wonderful info thanks!!!I do not think this is working because no one reported a small rhom growing to a 12 incher. Probably the guy knifeman was talking to just wanted to sell his small black piranhas. Dealers like used car dealers can lie to make a sell. I am just assuming.