Most experienced keepers of shoaling species will confirm that a single Pygocentrus will not be a lot of fun, not only because it is likely to remain shy and reclusive throughout its entire life and will hide all day except during feeding time, but also because it will show little to no natural behavior whatsoever. Two fish will also result in failure most of the time, because eventually they will turn against each other trying to establish dominance, and will often fight until one is killed. Even though there are exceptions to this, most piranha keepers will recommend three or more Pygocentrus. But there are no iron laws when it comes to keeping piranha's, which is demonstrated by various reports from Pygocentrus-owners: some recommend far more than 100 liters per fish, and others keep 8 or 10 healthy adult Redbellies in a 100 gallon (400 liter) tank... Important to remember is this: if your tank is cramped, you can expect stressed and unhealthy fish, and probably some casualties as well, due to disease and stress, aggression and territorial disputes. The more tank space your fish will have, the more comfortable and lively they will be. And piranha's, no matter what size or species, look best in a very large and natural looking tank to begin with.
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