Agree. This is simply put, not a community fish. To maintain relative peace within the group, it needs 12-15 and this requires your 30g tank. So this group, which I would up to 20 here, would be it for the 30g. In considerably larger tanks, with similar numbers, it can usually manage, but other species must be relatively active (no sedate fish), and no long-fin fish.
It is really unfortunate that this species is so widely available, because it has specific requirements and even then not always suited. Most of the other red/rosy fish in the genus (
Hyphessobrycon) that are closely related (likely the same clade according to Weitzman) are very peaceful, but not this one. The following from my online profile of the species may be of further interest.
Behaviour can be unpredictable; keeping the species in large groups and in larger tanks tends to lessen its aggressiveness. This variant behaviour, like the anal fin pattern mentioned above, may also partly be due to significant variations between the fish. The species has a large geographical distribution including areas of the Amazon, Parana, Guapore and Paraguay River basins. Dr. Stanley Weitzman (1997) has suggested that the "species" may be a complex of closely related species that are geographically quite variable over this wide area of Amazonia; it is quite possible that this "species" may actually be several different species, each endemic to specific river basins, but this will only be ascertained after collections from many locations have been studied in detail.
Most of the fish now available in the hobby are commercially raised and differ from wild-caught fish with respect to the dark shoulder or humeral patch. Commercially bred fish are descended from hybrids (perhaps unintentionally) of fish from different geographical areas, and the patch is shorter or all but absent on most; on wild-caught fish, this patch is black, elongate and slightly triangular. This decades-long inbreeding may also partly explain the fish's aggressive nature.
