Why do some deny the fact that corydoras DO sift through sand with gills?

[Rocky998]

I've seen this debate on here between two well known members and also debated among others here, as well as people on other sites. I don't understand how some who honestly do have years of knowledge with fish and aquariums can't recognize this as a normal trait. And with no disrespect intended with this question, its just an honest question, but if you do deny corydoras do this, have you ever kept these fish, and if so, have you ever sat there and observed them for long periods? I've seen people who literally deny it and claim that corydoras do not at all sift through sand, but the first thing my corydoras did when they got in the tank was use their gills to filter through the sand (you can literally see them picking it up in their mouths and then pushing it out their gills, it cant be mistaken).
There are also multiple videos online documenting this behavior.

This is not a post to start arguments, neither one to sound like I'm trying to bring this up in an argument argumentive tone. This is just something I've noticed and wanted to bring to attention with my own personal experiences and other facts.

Videos on YouTube recording said behavior:

 

[CassCats]

2 of those are slow motion videos

We as fishkeepers should aim to provide fish what they expect in the wild. This includes items that promote natural behaviors. In this case, sand for sand sifting species. Even if it's a small corner of an aquarium, they really should have the option to do so.

 

[GaryE]

Anatomically, Corys are filter feeders. That's structural.

They do put sand in the mouths. Fine sand. In nature, they are stream fish, and move through all sorts of substrates. People who have observed them in the wild report they hang around sand. It makes sense anatomically.

Can they survive and find food with gravel? It seems so. It helps that we add food that easily breaks into particles, making it easier to draw prepared foods into their mouths.

@CassCats slow motion videos show pretty standard earth eating/sifting. The combination of video evidence and anatomy seals the deal for me. They mouth and sift sand.

Do they need to? Only if we want to see somewhat natural behaviour. We can get around it with our foods that crumble into fine sand-like particles. Why we'd do that, I don't know. That's human behaviour, not Cory behaviour.

 

[MaloK]

They even swallow a small amount and it participate in their digestive process.

While there's no definitive scientific proof that Corydoras catfish intentionally ingest sand to aid digestion. The presence of sand in their poop has been observed by many aquarist.

And I think that it contributes to their digestive health, Like birds that swallow small rocks to help digestion. Sand looks like it acts as a natural grit in their digestive system, helping to grind down tougher food particles.

 

[AJ356]

Because they have already set up their tanks with gravel and can't face the truth. So sad to see corydoras when they are not in a sand substrate. I kept corydoras for years. Its a no brainer

 

[GaryE]

I think as @AJ356 said. While on this forum, I saw terrible attacks on our now late friend, Byron. He argued for larger groups of shoaling fish, and that angered some members here, who viewed it as inconvenient sentimentality. Everything Byron said about that fit with nature, and fit with research, which he cited. But it didn't fit with the fishkeeping plans of the men who got mad.

People who think they have the answers get mad at new questions.

 

[MaloK]

In 1976 I had my first tank.

I was 8 years old at the time. And aquariophillia was already a very widespread disease. It killed millions of fishes and created thousands of frustrated aquariums owners.

These old guys where saying everything you didn't want to ear. But... Listening proved it for me. And I listened.

Only beyond that point, real fish keeping begins.

 

[Back in the fold]

Byron was very science oriented and didn’t follow the crowd . I liked that Paul Loiselle quote he had in his signature . Can’t remember the exact wording but something like “ if you expect your fish to do well but won’t give them what they need then don’t cry when they die “ .

 

[Rocky998]

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2 of those are slow motion videos

We as fishkeepers should aim to provide fish what they expect in the wild. This includes items that promote natural behaviors. In this case, sand for sand sifting species. Even if it's a small corner of an aquarium, they really should have the option to do so.

Awesome videos showcasing that behavior!

Anatomically, Corys are filter feeders. That's structural.

They do put sand in the mouths. Fine sand. In nature, they are stream fish, and move through all sorts of substrates. People who have observed them in the wild report they hang around sand. It makes sense anatomically.

Can they survive and find food with gravel? It seems so. It helps that we add food that easily breaks into particles, making it easier to draw prepared foods into their mouths.

@CassCats slow motion videos show pretty standard earth eating/sifting. The combination of video evidence and anatomy seals the deal for me. They mouth and sift sand.

Do they need to? Only if we want to see somewhat natural behaviour. We can get around it with our foods that crumble into fine sand-like particles. Why we'd do that, I don't know. That's human behaviour, not Cory behaviour.

Absolutely agree with everything said!

Although, the last part, I do have one small gripe. Where you say: "Do they need to? Only if we want to see somewhat natural behaviour."
It shouldn't be a "want" for us to see, because it is a need for them. Gravel can be abrasive for their barbs and stuff like pool sand can injure them because of how coarse and jagged the pieces can be when they attempt to sift through it.
It's just natural for them to do and shouldn't be something taken away.

They even swallow a small amount and it participate in their digestive process.

While there's no definitive scientific proof that Corydoras catfish intentionally ingest sand to aid digestion. The presence of sand in their poop has been observed by many aquarist.

And I think that it contributes to their digestive health, Like birds that swallow small rocks to help digestion. Sand looks like it acts as a natural grit in their digestive system, helping to grind down tougher food particles.

That is super awesome if so!

Because they have already set up their tanks with gravel and can't face the truth. So sad to see corydoras when they are not in a sand substrate. I kept corydoras for years. Its a no brainer

Very true

I think as @AJ356 said. While on this forum, I saw terrible attacks on our now late friend, Byron. He argued for larger groups of shoaling fish, and that angered some members here, who viewed it as inconvenient sentimentality. Everything Byron said about that fit with nature, and fit with research, which he cited. But it didn't fit with the fishkeeping plans of the men who got mad.

People who think they have the answers get mad at new questions.

So true... I saw many people argue with him on things I'd consider to be basic knowledge. And some people were nasty towards him... Can only hope those people grow and realize that they were wrong.

 

[GaryE]

I haven't seen Corys try to pick up gravel. They'll take food from the surface, but I've never seen sifting of gravel. You can keep them on rounded gravel. They dig. I don't think there's any debate on that, You can keep them in tanks with no substrate.
"Can" is a wonderful word.
So is "why".
I like to think we want to watch our fish as living creatures, and as wonders of nature. They have adapted in so many ways to so many things. Corys sift sand. It's part of what they are. If you don't want to watch them be/act/live like Corys, there are realistic Cory fridge magnets you can buy instead of fish.
Cory behaviour is really entertaining.

I mean, you can travel, never leave the airport and say you've been to the cities. But why? It's the same with looking at fish and not watching them at their best.

 

[fishorama]

I have to disagree with the "have to have sand" thing just a little bit. Yes, it's more of a natural behavior for many fish, not just corys, but some cichlids & my fav loaches. But it doesn't wear down their barbels, if whatever substrate is clean! It's dirty substrate that does that. They can & do spit out the big bits & may sift the finer, smooth stuff or even foods out of their gills. That is a natural behavior too.

If we think we are keeping our tank fish "just as they live in the wild" we are sadly mistaken for many (most). Often, they live in huge "schools", hundreds of individuals not just 6 or 20+ we might have. We give them the best artificial home we can & learn as we go along the way. I bet most of us "could" do better given enough big tanks of awesome proportions, but the same as wild fish...not so much.

 

[Essjay]

@Back in the fold I've just looked in the Byron's legacy sticky in Tropical Discussion and the quote in Byron's signature is -

It is inhumane to deprive any animal of an element it regards as critical to its well-being, and totally naive to expect normal behavior in its absence. [Dr. Paul Loiselle]

 

[GaryE]

@Back in the fold I've just looked in the Byron's legacy sticky in Tropical Discussion and the quote in Byron's signature is -

It is inhumane to deprive any animal of an element it regards as critical to its well-being, and totally naive to expect normal behavior in its absence. [Dr. Paul Loiselle]

We have a long way to go to understand what fish view as critical. Linking aquarium fish to their natural needs is a recent aspect of the hobby. It has really been accelerated by short video, even though for Corys there is a famous vid where they are over road construction aggregate and someone claims they prefer the habitat like that. Stuff gets cherry picked, and people are always desperate to confirm their biases.

It'll take us time to get our hobby heads around that, if we ever do. I agree that it's a halfway effort at best. Keeping fish in captivity is kind of perverse and limiting, if you think about it (not that that will ever stop me). We're always going to see a reasonable facsimile of natural behaviour at best.

It doesn't mean we shouldn't try to gather knowledge and imitate habitat as much as we can.

Everything we do and how we do it is a product of history. Our hobby began as an ornamental fish trade, and developed a side direction as a fish behaviour watching pastime, and popular science thing. Both strands will go side by side and confuse each other, because when we discuss, we can be after different things. Some of us watch fish, some look at them, some breed them for themselves, some breed them to change them, some think they were made for us, some marvel in their evolution - we don't always make sense to each other.

 

[Magnum Man]

I find the title of this thread argumentative... I'm not going to dispute that Cory's sift through sand, I get the most interesting patterns in my white sand tank, from the Cory's, funny that I don't witness the same in my black sand tank... but my understanding, is they are not exclusively found in sand, in the wild, but also in mud, and leaf litter, and occasionally in rocky or gravel areas, unless this was witnessed as only a specific species, in those areas, it shows ( at least the wild ) that they also live in other types of substrate... perhaps there is more food in those areas, than in bare sandy areas??? I do have a small shoal of Serbi's in my South American Tetra tank, that has small rounded river gravel in it, and they have grown to maturity... this tank begs to differ, that they "need" to sift sand to thrive...

 

[WhistlingBadger]

I think people deny that cories sift sand through their gills because some people like gravel in their tanks better than sand, and we all like to feel good about what we're doing. I suppose it's the same reason some people deny that cories can do OK on gravel. The facts say otherwise.

I do agree with @fishorama that clean substrate is probably more important than gravel vs. sand. I've kept cories in a tank with lots of mulm and poor substrate-level water circulation. Despite the fine sand substrate, most of them didn't last long, and those that did lost their barbels.

I also agree with Gary that we aren't going to totally replicate nature in a glass box, and we don't always know what the fish consider "critical to their well being." I actually took issue with the late, great Byron on this a few times. Natural environments tend to be dynamic, and most popular aquarium fish are wide-ranging and at least somewhat adaptable. They don't exist in one and only one specific, static environment. So getting too dogmatic about a specific environmental requirement is often a bit silly. On the other hand, we must remember that in nature, most fish can simply leave if they don't like the environment. Our aquarium fish don't have that option, so it is important to give them the best environment we can. But the details can vary.

(The exceptions, of course, are fish that really do occupy an extremely limited, specific natural range. Such fish tend to not become popular in the mainstream hobby)

I'm not going to argue either point in the gravel vs. sand debate, because I don't particularly enjoy keeping cories, and all of my tanks have sand anyway, so I don't have a horse in the race. ha ha But it seems a strange thing to argue about. As long as someone is making an effort to give the fish a good, clean environment, with comparable tank mates, low stocking levels, appropriate temperature, water hardness, and cover/swimming space, I'm probably not going to argue with them about their specific substrate choices.