lautnusantara.com The idea of an invisible fish is quite strange, and people are often surprised when they first learn that such a fish exists—transparent, and almost invisible if not for its skeleton, like a tiny X-ray moving through the water.

Transparency in fish occurs in some species, such as the glass knife fish (Eigenmannia virescens) and the glassfish (Parambassis ranga), but it's extreme in the glass catfish, where only their skeleton and eyes can be seen. Their truly bizarre and otherworldly appearance seems like science fiction, but it is a scientific fact.

The true glass catfish is contained within the genus Kryptopterus, a group of 18 currently described species. However, it is suspected that many other species exist, and more may be described in the near future. These catfish are found in Southeast Asia and include not only several transparent species but also some that are only somewhat, or not at all, transparent.

Glass Catfish: Naming History

The glass catfish has a somewhat convoluted nomenclatural history. Many sources have listed it as Kryptopterus bicirrhis, a traditionally given scientific name, and almost all literature published before 2013 will use this name. K. bicirrhis, which we will refer to as the ghost catfish, most likely never made it into the aquarium trade in large numbers. It is distinguished from other members of the genus by having only a transparent head; the rest of its body is quite opaque and mostly white. They are a somewhat aggressive catfish and reach a larger size than the true glass cats, up to about 6 inches (15 cm).

It wasn't until 2013 that two authors, Ng and Kottelat, began looking at glass catfish and a misidentification was determined. They found that the true glass catfish represented two species, K. minor and a newly named species, K. vitreolus. Some sources published around the early 2000s would use K. minor to refer to all glass catfish. K. minor, as its specific name implies, is best known as the dwarf glass catfish. It is rarely found in the trade, although it is sometimes found mixed in with the true glass catfish. At only about 2½ inches (6 cm) in length, they are a much smaller species than the true glass catfish.

Our actual glass catfish is K. vitreolus, which finally found a scientific name in the 2013 paper. It is a larger fish than the dwarf glass catfish, but not by much, only reaching just over 3 inches (7.5 cm) in length. It is likely that almost all glass catfish in the hobby are K. vitreolus.

The body of the glass catfish is clear, with only its skeleton visible. Just behind the head is a silvery sac, which contains the organs. Sometimes these organs are slightly visible, especially the stomach after a large meal. Clearly visible is the prominent dorsal fin, which is referenced by the generic name "Kryptopterus," which essentially translates to "hidden fin."

Habitat and Behavior

Glass catfish are not very widespread, mainly entering the aquarium trade through Thailand. They are found in small rivers and waterways along the coast, where the water is soft and acidic. While many references cite high temperatures for these fish, they are actually found in waters with a temperature range between 21-24°C (low to mid-20s°C) and will thrive at a tropical aquarium temperature of around 24°C (76°F).

In the wild, they form large shoals of several hundred individuals, and while this is not possible to replicate in a home aquarium, they should certainly be kept in a sizable shoal. I would never keep fewer than half a dozen glass catfish and believe that a larger number is even better. They spend most of their time in sheltered areas, often dispersed, such as in plants, driftwood, and other decor. They will school up and move into more open areas, where they spend most of their time foraging.

Their natural diet consists of small crustaceans, insects, fish fry, and the like, but they will thrive on prepared foods. They have small mouths and will not bother other aquarium mates, although they may prey on small fish fry. More concerning is that they can be harassed and bullied by tank mates. Large and boisterous fish should be avoided in favor of peaceful and quiet species. Ideal tank mates are small tetras, rasboras, and other catfish, such as Corydoras spp. They also make ideal tank mates for some anabantoids, such as smaller members of the genus Betta, as well as small gouramis, especially the Parosphromenus species, or even various Badis and related species.

Breeding and Health

Breeding of glass catfish has been done in aquariums, but such reports are sporadic at best, and all of them are accidental. Information on their breeding is scarce and seems to be primarily from the "I had 12, now I count 16" variety. The fry are quite small and, like the adults, are largely transparent. As such, they can live in an aquarium for some time without being detected at all. Glass catfish are egg-scattering fish, and the eggs are laid indiscriminately throughout the aquarium. Beyond this scant information, nothing is known, and I don't expect many aquarists to succeed in breeding them.

Glass catfish are not commercially bred, and all fish in the trade are most likely wild-collected. Occasionally, some albino fish make their way into the trade. These very unusual-looking fish have red eyes and a golden sheen around their heads.

Like most catfish, glass catfish are highly susceptible to external parasitic infections, especially ich. They are scaleless, so many common treatments for these parasites can be deadly to the sensitive glass catfish, especially those containing malachite green. A strict quarantine regimen is recommended for these fish and all fish that enter their aquarium. High temperatures are well tolerated and should be the first line of defense against potential ich infections. Simply raise the temperature to around 88°F (31°C). Copper-based medications seem to be effective without harming the catfish.

Other "Glass Catfish"

While the name "glass catfish" almost universally applies to members of the genus Kryptopterus, two other groups of catfish are also sometimes referred to as glass catfish. Most notably, the rarely encountered members of the genus Parailia, particularly Parailia pellucida. Known as the African glass catfish, it looks like a glass catfish, except for a few dark lines along the body that somewhat spoil the effect.

While the body of P. pellucida is quite transparent, the fish can also appear to be somewhat opaque, depending on the viewing angle, lighting, and the fish's mood. At times, it is a pale white to silvery color. You will typically see a blurry image of what is behind the fish rather than the clear, see-through appearance of the glass catfish, like looking through a foggy windshield. On close examination, you will notice that the fish's main dark line is not a line at all but its blackened backbone.

The African glass catfish is larger than the Asian glass catfish, reaching a size of almost 6 inches (15 cm). Despite being larger, they tend to be more shy and will spend most of their time hiding in the aquarium. They must be kept in a shoal; a single fish will generally perish. They are typically found just above the gravel line and will forage for their favorite foods there. Ideally, their diet should include live foods, such as blackworms and similar fare, but they will often adapt well to sinking aquarium foods.

Just like the true glass catfish, their tank mates should be relatively timid and unassuming, though not too small to be eaten. Anything that will prevent the glass catfish from eating is definitely off the list. Instead, species like loricariid catfish or slow-swimming, medium-sized tetras should be considered.

They are found in the Nile River system and are typically in fairly deep and dark waters, although they adapt quite well to aquarium waters. They will prefer relatively neutral water and, as long as extremes are avoided, they are not fussy. Although they are rarely seen in the trade, these African glass catfish are an interesting addition to an aquarium.

Also from Africa, there is a second small genus of catfish known as glass catfish. Members of the genus Pareutropius often enter the trade, with the most common member known as Debauwi's catfish, the African glass catfish, and sometimes the African grass catfish. Pareutropius debauwi, like the ghost catfish K. bicirrhis, most likely has never been seen in the aquarium trade. Instead, the fish we know as Debauwi's catfish is P. buffei, more commonly known as the three-striped glass catfish. These two species are relatively easy to tell apart. P. debauwi has a single stripe along the body, while P. buffei has, not surprisingly, three.

Like the true glass catfish, the body of P. buffei is mostly clear, although this fish is a bit more opaque. They are torpedo-shaped mid-water catfish and are often found hiding in small groups in aquarium plants. They are found in heavily planted, flowing water in much of western Africa, including Nigeria, Benin, and Guinea.

As a smaller catfish, they only reach a total length of about 3 inches (7.5 cm). As mid-water fish, they are quite active and should be kept in small groups. They are slightly boisterous and can easily be mixed with small cichlids, such as members of the genus Pelvicachromis, as well as the usual mix of barbs, tetras, and the like.

Let's Get This Straight

The clear body of these fish, especially the glassfish in the genus Parambassis, is one of the most interesting things about them. However, for some reason, the practice of injecting dyes, often fluorescent, into these uniquely clear fish began. While this practice has largely disappeared in recent years, especially after the introduction of brightly colored genetically engineered fish that are not dyed or injected, the practice still continues. Dyed fish of several species are often seen in the trade.

Dyed fish are not dyed; rather, they are injected with a fluorescent dye, which fills the body cavities and gives the fish bright lines of color. To do this, the fish are taken out of the water, held out for a minute, and pricked with a large needle full of dye. The process is not sterile, and many fish suffer large wounds that become immediately infected. Even when the wound itself is not infected or provides an avenue for pathogens to enter the fish's body, the fish is highly stressed by the process, and these animals are far more susceptible to future disease outbreaks. This entire process has an extremely high mortality rate.

Ironically, even when the fish survives, the dye is temporary, and the customer who bought the fish is soon disappointed to learn that they have lost their color. It is not only potentially harmful to the fish but also harmful to hobbyists who expect their fish to last for a long time. In fact, when I was in a store, I more than once had to explain to customers that this was not the norm and that the brilliant colors on, say, cardinal tetras (Paracheirodon axelrodi) would not fade after a few weeks.

While the practice is slowly disappearing, such fish are still in the trade. Avoid buying them, and hopefully, the practice will disappear completely.

Ghosts in the Aquarium

There are some clear and tangible advantages to being an invisible fish. They are literally invisible to many predators, and we often overlook them in home aquariums. Once your eyes adjust and you can see the "invisible fish," they are definitely worth making room for in the tank.

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