The fishes of my childhood...The "Kuhli Loach.."

As a kid growing up in the aquarium hobby, I remember how incredible it was to see the enormous array of fishes that were available.

If I had to select one fish that pretty much got me into the hobby, believe it or not, it'd have to be the "Kuhli Loach", Pangio kuhlii.

It was exotic, fun, and everything I felt a tropical fish should be.

And quite frankly, it was the first example of a fish that I collected which I hadn't a damn clue how to really keep it correctly. By some miracle, many my Kuhlii seemed to survive despite my ineptitude. In more recent years, I've come to really understand this fish more by studying not only the fish and it's ecological adaptations, but studying the natural environment from which it comes, as well. 

Now, in case you've truly been living under a rock and don't know about this fish, it's a slender, almost "eel-like" fish, with largely nocturnal habits and endearing behaviors. They top out at around 4" (10 cm), so they're well-suited for modest-sized aquariums.

Although they look superficially like eels, they are most definitely not. Eels have  fused pelvic, anal, and caudal fins. Kuhli loaches have distinct pelvic fins and their dorsal, caudal and anal fins are distinct and separate.

(Image by Robert Mollick, used under CC BY-SA 3.)

We tend to call them "scaleless" fishes, because they have really small, widely-spaced scales, giving them the appearance of being scaleless. That being said, they are a bit sensitive to medications and such, much in the way truly scaleless fishes are, so use caution when exposing them to medications or salt, etc.

This is also another reason why you should keep them in aquariums with softer substrates, like sand, or mixed botanical substrates. Not only will this help prevent injuries which may lead to infection, it more accurately replicates the substrates found in the wild habitats from which they come- the environments which I believe we should be replicating when we keep these unique fish.

(Cool fish. Not cool substrate choice.)

Of course, like many fishes, this species suffered over the years from a lot of taxonomic reshuffling, and it's been known by the pseudonyms, Acanthophthalmus kuhlii and Cobitis kuhlii...A real mess! The species name is an homage to zoologist Heinrich Kuhl (an interesting character who died way too young..One could only imagine the discoveries he could have made had he lived longer...).

Oh, and there are a bunch of very similar-appearing species to simply add to the confusion. Pangio kuhlii may actually represent a "species complex" and commonly used "synonyms" include Acanthophthalmus kuhli, A. semicinctus, and Pangio semicincta.


And, hobbyists being hobbyists, we've lazily referred to this fish as the "Coolie" Loach for generations, because...well..because we're lazy, and because, apparently, it's easier to spell "coolie" than it is to spell "kuhlii" or "kuhli" for some reason...😆

The meaning of the genus Acanthopthalmus is 'thorn' or 'prickle-eye', after a spine beneath each one of the fish's eyes, a strong homage to the anatomical protection afforded them for their habit of burrowing into soft substrates, like sand, mud, and leaf litter.

That sounds good!

Who in our world doesn't love a fish morphologically adapted to burrowing into leaf litter?🤓 Now, the burrowing habits of this fish don't endear them to everyone, of course. Planted tank people are anything but excited about the roots of their precious plant specimens being exposed! And these guys tend to be rather nocturnal in nature, so you're likely to only catch glimpses of them most days. You often see them comically and endearingly  buried in the substrates, with only their heads poking up. 

Oh, and they're a bitch to net. Don't believe me- ask anyone who's ever worked at the LFS who's job it was to catch the wiggly little bastards! 

Kuhlis are social, gregarious fishes. Shoaling fishes, more specifically. They categorically are best kept in group. I wouldn't even consider keeping this fish if you don't have the ability to accommodate at least 4-5 of 'em- or, more beneficially, up to a dozen or more. Gregarious, yup!

(Image by Marrabbio2. Used under CC BY-SA 3.0)

However, they are not fishes that you're likely to see out at all hours of the day in a brightly-illuminated aquarium, dancing in the current or rushing to the front glass in search of food for all to see. This reclusive behavior makes them a fish that really, truly deserves an aquarium of their own. If sufficient food doesn't reach the bottom where they dwell, or if food sources aren't available within the substrate itself, these fishes can easily starve to death. 

You may occasionally find one in a filter intake or overflow, waiting for you to rescue it. This is a typical occurrence with these guys, and a very sure indicator that some heavy-duty "loach partying" was going on in the tank the night before! Checking those "weird spaces" each morning is just part of the daily ritual when you keep these guys.

Oh, while we're talking about the Kuhli's morphology and how it takes advantage of it,  let's take a second to whack us hobbyists upside the head one more time! For some bizarre reason, this fish is popularly relegated to the "role" of "scavenger."  Like, WTF? Does EVERY fish with reclusive nature, bottom-dwelling habits, and nocturnal preferences have to be branded a "scavenger?"

(NOT a scavenger. However, it lives on the bottom of the tank...😂)

In the wild, they tend to eat small crustaceans, insect larvae, and sometimes fish eggs....A similar diet to many other fishes that we don't anoint as "scavengers", right? Like, Tetras..I mean, are Cardinal Tetras "scavengers?" Damn, what the hell is wrong with us? Now, granted, they'll often ingest a bit of substrate to sift through for crustaceans and such..but this benthic feeding behavior does not mean that they are "scavengers." 

Stop thinking of them as such. Just don't purchase these fishes with the intent of utilizing them to "keep the tank clean." That's not the fishes' responsibility It's yours. And, YOU have a damn siphon hose for that! Don't like that responsibility? Cool. Just take up collecting sneakers or something. 🤬

"Okay, deep breath, Fellman." Early morning rant over!

Of course, I'm fascinated by this fish, but I'm even more fascinated by its environment. Of course, the habitats from which our fishes come from always seem to dictate their morphology, right? So one can learn a lot about a fish by learning more about its habitat!

Geographically, the fish has been collected from a broad range of locales, as wide apart as Indonesia and Malaysia. Oh, and Thailand, In fact, I even stumbled on a work entitled  "Peat swamp fishes of Thailand." from 2002 which referenced it's occurrences there.

The environments that this fish is found in are many and varied. Fishbase references "hill streams to lowland forest canals and peats." Of course, the "peats" part fascinates me. A reference to the peat swamp forests that we've discussed almost irresistible subject for a proper botanical-style aquarium!

And, as we all know, the substrates in many of our fishes' habitats directly influence the chemical environment. Of course, the habitat of the Kuhli is no exception. My research revealed that the fish is found in water with pH ranging from 5.5 - 6.5. 

It's also indicated that it comes from Shallow, sandy streams with fine cobbled rocks and a few branches and leaves.

Seems like there are always leaves, huh?

I think the real fun with these fishes can be had when we make anattemtp to replicate as many aspects of their natural habitat as possible- specifically, the substrates. 

In peat swamps, the peat layers may be well in excess of  3 feet (1m) deep. The floodplain forests are found along rivers, streams, coasts, and lakes. The seasonal flooding inundating the forests for short periods leads to an influx of sediment and mineral enrichment during high water periods. 

These soils are best replicated by using "non-traditional" substrates, like...coconut-based materials, finely-crushed botanicals, mud, sediments, etc...


And I also feel very strongly that the Kuhli, like many fishes with benthic lifestyles, can benefit from being kept in an aquarium designed and configured to provide feeding opportunities within the system itself, to supplement any foods that we're offering.

Many fishes, like Headstanders, characins, and others, simply consume tiny crustaceans as part of their sediment feeding activity. Now that we're more likely to set up aquariums with fine, silty sediments stocked with tons of little copepods and worms and such, these experiments may yield very interesting results!

It is absolutely possible to create a real "active substrate", filled with these creatures, and to be able to "pre-stock' it with cultures of small life forms prior to the introduction of fish! And of course, there are ways to replenish the population of these creatures (and even the substrate itself) periodically, resulting in extremely productive systems, too!

An interesting experiment to think about, huh? Even more interesting to actually execute. Such a system, with a heavy "substrate-centric" focus can be successfully managed long-term- resulting in a terrific environment for Khulis and others.

And of course, the whole idea of a rich, sediment-sand-and-soil substrate enriched with botanical materials is completely in line with the "best practices" we've developed as a community to create dynamic, botanical-style aquariums. In our case, not only will there be an abundance of trace elements and essential plant nutrients be present in such a substrate, there will be the addition of tannins and humic substances which provide many known benefits for fishes as well.

The best of both worlds, I think!

Now, sure, this tank will probably have some suspended material in it at all times. The water may not be 100% crystal clear, because of the Kuhli's benthic habits. It goes with the territory, and it's simply something we need to understand and appreciate when going the more natural route.

These dynamic habitats are not difficult to replicate in the aquarium. We need to understand that they play a functional and aesthetic role in the overall aquarium, as we've touched on many times here. Realizing that placing leaves and botanical materials on the bottom of the aquarium is not simply making an aesthetic statement. Rather, it's an homage to the function of the dynamic habitats we love so much.

I am 100% certain that this is the key to keeping fishes like our friends the Kuhli loach extremely happy for extended periods of time.

To me, that's the most amazing part about keeping the fishes of my childhood: Merging the excitement and fascination they bring with the art and science of providing as accurate a representation of their natural habitat as possible. 

Stay creative. Stay studious. Stay persistent. Stay observant...

And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


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