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The Nice Puffer: Colomesus asellus, the South American Puffer


By Neale Monks

Pufferfish generally have a reputation for being aggressive loners, but one species that isn't is the South American pufferfish Colomesus asellus, often simply called the SAP.

The explanation for this surely comes from its distinctive mode of reproduction. Most other freshwater pufferfish guard their eggs and fry, but South American pufferfish do not. They simply scatter their eggs onto the substrate, and the fry, once they emerge, spend a period of time floating down the river as plankton. Consequently South American pufferfish have no reason to be territorial. In fact they are not tied to one particular place at all, and instead migrate up and down rivers, often moving into lakes for part of the year.

Indeed, being such active swimmers makes South American puffers very distinctive in terms of aquarium maintenance. Things like caves and other hiding places are largely irrelevant. While they certainly like a quiet corner behind a plant when they're sleeping, these fish are otherwise constantly on the move. So aquarium space and water current are much more important than is the case with other, usually less active, puffer species.


Older books sometimes describe the South American pufferfish as Colomesus psittacus, a name that actually belongs to a related species that lives in estuaries and shallow marine habitats. The correct scientific name for the South American pufferfish is in fact Colomesus asellus.

The two species can be distinguished by size (Colomesus psittacus being much larger when mature) and colouration. Colomesus asellus has a black patch on the underside of the caudal peduncle, a feature not seen on Colomesus psittacus.

Colomesus asellus
Colomesus asellus


Fishbase and other academic sources report a maximum length of 15 cm, but aquarium fish never get that big; 7-8 cm (about 3 inches) is usual.

Water chemistry and quality

South American pufferfish are found in a variety of habitats, from quite soft and acidic rivers like the Rio Negro through to the estuary of the Amazon River. In other words, water chemistry itself is not particularly important. Anything between pH 6-8, 5-20 degrees dH will suit them well. While not common in brackish water habitats, they will do well in low salinity systems up to around SG 1.005.

On the other hand, as with all pufferfish, water quality should be good. By pufferfish standards, South American puffers can be considered hardy, and they are certainly less sensitive to nitrate than things like Tetraodon mbu. But that doesn't mean that they will put up with bad water quality for long. Generous filtration and regular water changes are essential. Taking into consideration their need for strong water current, canister filters (either internal or external) offering water turnover of 6-10 times the volume of the tank per hour are in order. Change 25-50% of the water per week, aiming for less than 20 mg/l nitrate if possible, and certainly no more than 50 mg/l.


Fishbase reports that these puffers consume crustaceans, fish, plankton and plants. In the aquarium they are omnivorous and will take a wide variety of foods. My specimens enjoy bloodworms, Physa sp. tadpole snails, chopped shellfish (prawns, squid, cockles, mussels, etc.) and live foods such as Daphnia, earthworms and woodlice. Individual specimens vary in their interest in plant material, but at least some specimens seem to enjoy Spirulina flake, algae wafers, and frozen peas.

Social behaviour

Unlike almost all other freshwater pufferfish, South American pufferfish are not territorial and exhibit no aggressive behaviour towards their own kind or other species. In fact they are nervous when kept singly, and become much less "neurotic" when kept in groups. During the daytime fish will follow one another around briefly, often squabbling over food; but at night (or if alarmed) they will settle on the substrate as a group.

Ideally, keep South American puffers in groups of three or more specimens.


In the wild breeding seems to be similar to that of pelagic pufferfish, with the parents exhibiting no broodcare at all. The eggs are relatively small and the fry are planktonic. Breeding behaviour has not been (knowingly) observed in aquaria, and in all probability rearing the fry will be very difficult.


In broad terms these fish are hardy, and when sick respond well to medications (such as eSHa 2000 for finrot and fungus) and heal quickly.

However, two aspects of healthcare need mentioning. For whatever reason, South American puffers are very prone to whitespot (ick) and are often among the first fish to show the symptoms such as "flashing" against rocks and the white cysts on the skin. Treatment needs to be prompt if serious problems are to be avoided. I have used eSHa EXIT to treat whitespot on South American puffers without any problems, but because puffers tend to be sensitive to certain chemicals, it's a good idea to observe your fish closely during any treatment. Alternatively, adding tonic salt to the aquarium at a dose of 3 grammes per litre and raising the temperature to around 28 degrees C should kill the parasites without doing the pufferfish any long-term harm.

The other major healthcare issue with South American puffers is the rate at which their teeth grow. Even by pufferfish standards, this species has peculiarly fast-growing teeth. While some hobbyists (myself included) have managed to slow down tooth growth by manipulating the diet so that it contains crunchy foods like snails, in reality most aquarists will find themselves needing to "trim" the teeth one or more times per year.

Essentially this involves sedating the fish by placing it into a container of water where clove oil (eugenol) has been added at a dose of 2-4 drops per litre. After about a minute the puffer should be drowsy, meaning it can be handled without squirming. Using wet hands, grasp the fish carefully, and use cuticle clippers to nip off the points from the upper and lower parts of the "beak". Once you're done, put the fish back into the net, and hang the net in the aquarium so the fish is bathed in the water current. After a short while it will be as sprightly as ever, and ready to be released.

While this might sound a bit nerve-wracking, it's actually pretty easy and a lot less stressful for the fish than having an overgrown beak that prevents it feeding properly.


Whether or not the South American pufferfish is a good community fish can be argued both ways. On the one hand, it is not aggressive and it doesn't view live fish as food, except perhaps livebearer fry But on the other hand the South American puffer is a confirmed fin-nipper. When hungry it will view the fins of slow-moving and long-finned fish as food. While not unusual in this regard (many tetras and barbs behave the same way) this does mean that it can't be kept with many of the most popular community fish. Angelfish, gouramis, Congo tetras, livebearers, Corydoras and so on are all likely to be nipped.

In my experience, the safest tankmates are those that hide all day (like Synodontis catfish) and those that are very fast swimmers (such as glassfish and bleeding heart tetras). But if this approach is to work the tank will need to be reasonably large so that these fish can keep out the way of the puffers.

The ideal situation though is surely to keep a group of South American puffers by themselves. Being nicely coloured, constantly active, and not too big, a pack of half a dozen specimens would look great in a 125-180 litre/33-44 gallon tank.

Final thoughts

Often pushed as the "community tank puffer" this is only fair up to a point, and these are still nippy fish at time. But they are quite easy to keep, hardy, and generally very well behaved aquarium fish. For the aquarist looking for a pufferfish species that is exciting to watch as well as attractive and interesting, it's hard to think of a better species than the South American puffer!


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