Jennifer O. Reynolds, M.A., and Nora Hickey, D.V.M.
Nora and Jen weigh in on the ongoing debate about whether cory catfish, including Hoplisoma species, pygmy corys, and other popular species, truly need sand in their aquariums. They also provide expert recommendations on the best sand options for Corydoradinae catfish to ensure their health and comfort.
Nora: Jen, in honor of your recent re-entry into the world of cory catfish keeping, let’s talk about my favorite fish today! One of the most common questions I have seen about these fish is whether they need sand.
Jen: I’ve seen this come up on a number of groups and forums, too. I notice that those conversations seem to focus on whether or not cory catfish—the common name for the group of catfish in the subfamily Corydoradinae, which were almost all in the genus Corydoras prior to a recent important taxonomic revision—are always found on sand in nature.
But I think that focus misses a crucial aspect of answering the question and can also be misleading in how we evaluate this when designing our aquariums that feature or have cory catfish in them.
Can corys survive in an aquarium on gravel or other substrates that are not sand? Yes. Will they be thriving and able to engage in the full suite of sifting and foraging behaviors available to them? No. As an aquarist who enjoys fish behavior more than almost any other aspect of fishkeeping, I want to ensure all my fish have appropriate environments that allow them to express their full range of natural behaviors, at least within the bounds of a reasonably sized aquarium, while also doing the best I can for their long term health.
In terms of natural habitat, corys are most often found associated with sandy or silty substrate. While they might occasionally be found on substrate with larger rocks in it, remember that they have freedom of movement in the wild, so seeing them in a rocky area does not mean that’s an ideal environment for them for the long term in an aquarium. If the sand or gravel used as a substrate in their aquarium is too big, their sifting and foraging behaviors are impacted because they can’t take the substrate into their mouth and pass it through their gill rakers, which is a pleasure to watch and seems like an enjoyable activity for these busy fish!
Nora: And the sifting behaviors are just so cool to watch! They swim around on the bottom hunting for food by moving their delicate barbels into cracks and crevices. Catfish’s barbels (and the rest of their skin) are covered in taste buds, which I think is a very appropriate trait. Then when they find something they like, they suck it in and stop swimming, sitting completely still while they grind it up. If you look closely, you can see their mouths moving really quickly while they’re doing this–adorable!
Jen: Health-wise, what impacts have you seen to corys kept on sharp substrate or gravel?
Nora: The biggest concerns I have about barbel health are trauma and poor hygiene. Trauma is any kind of injury the barbel might sustain from the environment (or even other fish nipping). The barbels are covered in soft skin that can be damaged by rough or abrasive surfaces. This is why fine, smooth sand is so important. Poor hygiene, such as an environment with a lot of leftover food and feces, can predispose fish to infections with water molds and bacteria. Maintaining a clean environment is fundamental to preventing health problems with any fish.
Jen: Some of the wild-caught Hoplisoma I recently received arrived with eroded barbels, presumably from damage throughout the process of capture and transport, and maybe poor hygiene in certain phases of their shipping journey. I can see they are starting to slowly recover though, because the barbels appear to be growing longer after several weeks on fine sand with clean water.
Nora: Pathogens can definitely take advantage of trauma like what you described for your fish.
Anything that disrupts the protective skin and mucous barrier can make them vulnerable to invasion. Fortunately, cory cats are tough little fish and maintaining a clean environment with proper substrate is often all their barbels need to recover from damage. But of course adding a little salt won’t hurt!
Jen: One of your recent arrivals had water mold problems on their barbels, didn’t they? I have seen this on stingray mouths and tails too. It seems that proximity to the substrate is a risk factor for this because they’re closer to decaying food bits or other detritus that can have pathogens in them.
Nora: Yes, fish that search for food on the bottom of an aquarium are definitely more at risk of encountering moldy food bits! I received some corys awhile ago that had an infection of their entire branchial cavity with Saprolegnia (a water mold).
Jen: What counts as the branchial cavity?
Nora: It’s the fish’s mouth, including the gill arches. I can see how picking up a rough substrate in their mouths and sifting through it for food could cause some significant damage to the lining of the mouth. I don’t know why they had that infection originally, but some of them did recover with a clean environment, salt, and TLC.
Jen: That’s interesting. So it’s not just whether or not the substrate can pass through, or whether the outer areas of the barbels are exposed to sharp or abrasive substrate surfaces, but the entire branchial cavity could be impacted by inappropriate substrate choice.
Nora: I think so.
Jen: So having soft fine substrate for cory cats is important for them from both a health perspective and a behavioral welfare perspective. They’re very happy-go-lucky little fish in general. I really enjoy watching them sift sand and forage. They’re not picky eaters, so that part of their husbandry is pretty straightforward compared to some other fish I keep that require live foods.
Nora: Yes. As long as you have some food that reaches them on the bottom, they will be happy no matter what it is!
I should also mention that because of the anatomy of the barbels, I suspect other systemic conditions (like sepsis and inflammatory processes) can impact the barbels. Systemic bacterial infections can cause disruptions to the blood supply that might make the tips of the barbels die. So unhealthy barbels can be caused by things besides inappropriate substrate. Looking at barbels is a great way to evaluate your cory catfish’s health. Long, luxurious barbels that are being used to search out food are a sign that your cory cat is doing well.
Jen: So, we have a favorite sand for our cory cats. It’s a very fine sand and quite similar in size and appearance to the sand of the Rio Negro area where you observed catfish and where we caught and photographed some.
Jen: It’s the Caribsea Moonlight Sand, and it’s so fine. The grain size is 0.25-0.75 mm.
Nora: And it is super smooth. It doesn’t even feel like sand when you touch it! This is a small enough sand that cory catfish can sift through it, and we haven’t observed it to cause barbel problems in any of our fish.
Jen: I’ve been wondering about what folks can do if they have an aquascaped community tank with plants. Sometimes there is a specific substrate they want to use to grow their plants and the focus of the tank isn’t only the cory catfish (Imagine that!? Haha!) Do you think having at least some areas of fine sand for the cory catfish, even if there is another non-abrasive plant substrate, is a reasonable compromise? I know a lot of people use areas of decorative sand in their aquascapes.
Nora: Good question, I hadn’t thought of that. I would expect the cory cats to figure out pretty quickly which area of sand was good for sifting if they were in a tank with mixed substrate types.
Jen: That is because you design your tanks entirely for the comfort of your catfish, which I am sure they thoroughly appreciate.
Nora: I can’t keep a fish tank without catfish! I think it’s a nice compromise to have a little fine sand for them to enjoy sifting in—although if they had jumbo jets like my CW051s, I doubt the sand would remain in its designated area long!
Jen: Yes, even my group of smaller Hoplisoma spp. have managed to move the Moonlight Sand toward one end of the tank over days of sifting and slowly bury the dried leaves I’ve added.
Some of the smaller cory species would probably do fine with a decent sand layer on top of a plant substrate too, which is another technique some aquascapers use. In any case, avoiding sharp substrates and providing some small, smooth substrate that can be effectively sifted through their gill rakers is going to enhance their overall wellbeing in our aquariums.
Nora: And there is nothing like a tank full of happy cory cats!
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