I've maintained pencilfish for many years, but they are not always easy to get as you're finding out. Don't know why, they are wonderful little tetra-like fish and very colourful.
There are several species, found in two genera as established by Gery, Nannobrycon and Nannostomus; you may still see a reference now and then to Poecilobrycon, but Gery determined that this is probably a subspecies of Nannostomus. There are two known species in Nannobrycon, and several in Nannostomus. In all cases, these fish are shoaling characins and must be kept in a group of minimum 5 or 6, more if space permits. The males will challenge one another regularly, but seldom cause any damage; but for a small fish, they are quite feisty at times. Some species have an adipose fin (a characteristic of many characins) while other species do not, and occasionally fish within the same species may or may not have this fin. All species have distinctive diurnal colour patters, meaning that they look quite different during darkness than during the day. Those species posessing lateral strips (all but one, Nannostomus espei
, have this pattern) lose the stripes at night and have broken blotches along the sides, and different colouration. Gery suggested that the Nannostomus genus is still actively evolving, and there are indications that the species in this genus exist in "pairs" that are indistinguishable externally, and this may explain why fish within some of the species can be somewhat variable (e.g., the presence or absence of the adipose fin in the same species). The "pair" species are allopatric, meaning they live in different localities and not together.
The two species in Nannobrycon, N. eques and N. unifasciatus, are easy to distinguish from the others because they both swim in a oblique manner (head up at about a 45-degree angle). The species in Nannostomus swim normally (horizontally) and are the more colourful.
The most common species is Nannostomus beckfordi
, and it is the hardiest of the family and the largest in size though only 6 cm (2.2 inches) maximum; it is well able to take care of itself in any community tank that does not have aggressive fish. It mixes well with most of the other characins (tetras and hatchetfish), catfish (corys, farlowella, whiptails, small species plecos), dwarf cichlids (an ideal dither fish), etc. The sexes are easily distinguished except in young fish; males are considerably more darker, a beautiful reddish brown, and very bright red on the fins with ice blue tips on the ventral fin pair. The males almost constantly challenge each other, sometimes three in a line, side by side, darkening to almost black, but nothing rough ever comes of this; much like the male Black Phantom tetras, and some of the other Hyphessobrycon species. And N. beckfordi will spawn very readily and frequently. The only negative is that the males will drive the females very hard, which is common to many of the characins actually, and sometimes the female will die as a result. It is therefore better to have a ratio of more females to males with this species, and other pencilfish for that matter. That allows the males to divide their attention at spawning times. I have this species spawning regularly in my 115g aquarium, although the eggs are gobbled up by the other fish as soon as they are expelled.
The remaining species in Nannostomus are a bit less rambunctious and are much the same in terms of care and behaviour. N. beckfordi is now farmed commercially, so most fish in stores will be tank raised. The other species are largely wild caught, which means you have to pay closer attention to water parameters. They all come from very soft and acidic water. N. marginatus, the Dwarf Pencilfish
, occurs throughout the Rio Negro system, found in company with cardinal tetras, hatchetfish (Carnegiella marthae
and C. strigata fasciata) and many other species of shoaling tetras. The Coral Red Pencilfish
, discovered during the last decade and thought initially to be a colour variant of N. marginatus [along the lines of the "pairs' I mentioned above], was established as a separate species, N. mortenthaleri, in 2001 by Paepke & Arendt. This fish is found in streams of the Rio Nanay in Peru, the country home of your panda cory, although not in the same watercourses. Another very closely related fish, also originally thought to be a variant of N. marginatus, was described by Zarske a couple of months ago as a new species, N. rubrocaudatus, an incredibly beautiful purple variation of the two afore-mentioned species.
The other species are less often encounterd in the hobby, unfortunately; they are fascinating fish to my thinking, and easy to maintain provided water quality is good and constant.
FishinPole has a group of N. mortenthaleri and may have further insights; I only acquired mine last month.