Substrate observations

[raylove]

I recently bought 10 (false) julii corys, the first time I've had corys for over a decade. Back in the day I didn't know any better so previously I've only kept groups of 4 and only ever with gravel substrate. This time I did it properly, bought a larger group and converted about 20% of the floor of my 55 gallon to sand.
2 weeks in and these are my observations:
For the first couple of hours they kept together in a group but once settled there are now 4 that hang out together most of the time but the other 6 are loners - they forage, glass surf and rest pretty much alone. I guess in a tank they are close enough to be aware there are others close by, but I'm not seeing the social interaction I was expecting.
The biggest surprise though is the substrate. They almost never venture on to the sand! Even when swimming from one end if the tank to the other they seem to actively avoid the central sandy area by swimming around it. I have only seen one fish briefly forage in the sand just once - certainly no 'sand sifting' - whilst on the other hand, they poke around in the gravel all the time. My impression so far is that given the choice, they seem to prefer the gravel.
I'll give it until they mature but tbh I don't like the look of the sand as much as I thought I would, so if things don't change I'll probably ditch it.
This is quite interesting too:

 

[CassCats]

First off this video is proven to just be corydoras in a transient waterway, they migrate between suitable habitats. This video is full of misinformation and Ian Fuller has also disproved this video particularly.

Second, if there's more cover on the gravel side in terms of plants and decor, they'll choose that as 2 weeks in is still settling in and are likely to feel more uneasy about open spots.

They should always be given the option to sift, regardless. Gravel traps more bacteria under it, and while most fish don't come into contact with the bacteria level there, corydoras who dig in substrate to eat, DO. Even a small cut on their barbels comes in contact with this bacteria and in turn it becomes infected and the fish lose their barbels. It's why gravel is a poor substrate choice for them health wise. It's not just a matter of it being sharp or whatever not, it's a matter of other factors.


Hoplisoma trilineatus are a slightly more cover-happy cory species. They aren't huge on spending a lot of time in exposed areas, they'll rest under wood and among plants. Even leaf litter if you add that as well

 

[TwoTankAmin]

 

[MaloK]

Water flow is the key.

Any Cory need 3 different layers of possible activity... Not that easy to achieve in small tanks.... Any imo.

But that all relies on water current.

1: slow, more near to not moving water, in the center area. Cories still requires a lot of rest, need some quiet places. And a lot of exits where they hide.
2: medium, near the bottom, where you feed them, cool places to be, facing a slow persistent front current...
3: high, near the top or middle of the tank, Where they can compete and have fun playing with each other...

If you can achieve the water movement required and enough resting place at the same time.

Boom !

Don't worry about substrate, they are going to get used to everything, and have their preference.

You're giving them too much choice.

 

[raylove]

Thanks for the responses. I'm absolutely going to leave the sand in place until they are fully settled/mature as I mentioned before, because I know 2 weeks isn't long.
Yes, the tank is heavily planted so the sandy part is more exposed, however, they aren't hiding in the plants, they are all over the tank, including a gravel area that is in fact more exposed than the sandy part.
I totally agree about bacteria which is why I gravel vac at least once a week, twice if I can. Assuming gravel isn't sharp (mine isn't) I think most of the injuries we see to barbels is down to bacterial infection rather than physical damage.
I suspect the reason they seem to prefer the gravel is precisely because it has more gaps in it where they are finding food. I've just been surprised that they avoid the sand so much; they don't even try to find food there.
I don't think anyone can fault the logic of why sand is good for corys. However, I had imagined that they would head straight for the sand and exhibit the 'natural behaviour' that is often mentioned, but it seems that given the choice that particular aspect may not be such a big deal. I'll keep watching!

 

[Colin_T]

It could be the colour of the sand vs the colour of the gravel. If the sand is white and there are no floating plants, it will be too bright for them and they will hang out on darker coloured substrate instead. You could try watching the fish after lights out and see if they use the sand then. If they do, that would confirm too much light over the sandy area.

 

[raylove]

Good point, the sand is yellow play sand and the gravel slightly lighter than 'standard' gravel but the sand is certainly lighter. However, the whole tank is fairly shady as I have lots of vallis trailing over the surface (there is enough room for them to reach the surface and gulp air if they need to, but that was a consideration!) so the contrast is modest.
It's late morning here so the lights are off. I can only see 4 of them at the moment - two are glass surfing, two are rooting around in the gravel.
Ha ha - just as I typed the above, one of the ones that was rooting in the gravel swam across the sand without stopping and carried on digging in the gravel on the other side

 

[GaryE]

The gravel is a food trap. It makes more sense to root there because they are finding something. On smooth sand, most filtration moves fallen food to a rougher surface - the gravel.

I set up Cory tanks with sand, and no gravel. I've never understood the golf course type set up, with sand traps. The fish will react to open areas as dangerous, and because of the current, the sand will be a desert. Why sift if the food isn't there?

I have a 120, 6 foot tank with one end, 2 feet, gravel. The rest is a mix of pool filter sand and playground sand. In that 4 foot stretch is where I find the Cory group fish. They are never up in the gravel area favoured by some of the tetras. The flow in the tank carries food to the 4 foot sand side. The sand is gradually burying the gravel as it quietly shifts, and that's fine by me.

They spread out and forage, always keeping an eye on each other. They come together sometimes, often at night, but their social life is easy going. I have a decent group of melini and a smaller group sold as brevirostris, and they tend to run together. but sleep in separate groups.

As for the wild shots, first, @CassCats mentioned Ian Fuller. He is a Cory researcher who runs a superb, scientifically grounded website on the fish. I have friends and acquaintances who have studied Corys in the wild, both Brazilians and regular visitors to the region. Corys are everywhere, in many habitats, as they live in a region where seasonal changes are huge. Water levels go up and down, lakes dry and refill, rivers are cut up into ponds waiting for the rains, forests flood - it's a dynamic part of the world if you live in water. Small catfish migrate looking for ideal areas. Sure, they spend time over gravel and rocks. I spend time on highways, but I don't live on them. They get from point A to point B, and as it dries or floods, react accordingly.

 

[raylove]

Quick update one month on. The corys still seem to actively avoid the sand. Only twice have I seen them nose in it and on both occasions they seemed to choke and flashed against the substrate for a minute or two afterwards. When I first introduced the sand my denisoni barbs did the same thing, so I wondered could there be an issue with the sand? Possibly, however it is soft play sand from a source frequently recommended on this forum, and also I used sand from the exact same bag in a tank where I am temporarily housing some pygmy chain loaches (temporarily has actually been over 6 months so far)! Those fish are fine - I have never once seen them forage in the sand but they certainly aren't distressed by it. I've had them 8 years and previously in a gravelled tank they were always digging around.
As has been said, I guess fish will only dig if they expect to find food. However, surely this also means that the 'natural behaviour' which they 'enjoy' is actually something they do only if they have to? I've given mine the choice and they've voted with their fins - they prefer the gravel and seem to have no interest in sand sifting just for the love of it!
Obviously sharp angular gravel is going to be abrasive, so we must avoid that. However, it got me thinking - is sand really softer than rounded gravel? Even 'soft' sand can be abrasive - I won't walk barefoot on a beach as the sand chafes my feet. Barefoot on a pebble beach is painful but the round stones don't actually make my skin sore once I step off of them. I'm thinking that maybe catfish barbels are no more likely to suffer physical damage from rounded gravel than they are from sand? Also, are we really sure that sifting potentially abrasive sand through delicate gills is actually a good idea, regardless of whether or not it happens in the wild?
These are the first corys I've had for maybe 15 years and previously I kept them with rounded gravel as I don't recall the sand opinion being a thing back in the day. Before then I'd kept them many times going back to the 1970s and I had never had a problem with infections or damaged barbels (even in the early days when I probably didn't choose the gravel as carefully as I would now)! I therefore remain somewhat unconvinced about the wearing away or physical damage caused by gravel - bacterial infection caused by poor husbandary seems more logical to me, and I accept that might be more likely to occur with gravel if not cleaned regularly.
Anyway, I've now given up on the sand and removed it. I wasn't really pleased with the effect, and cleaning was more difficult as I find sand difficult to vacuum. With the Cory's steering clear of it, it was serving no purpose, and within a day or so of taking it out they began using that part of the tank too.
I know I will be in the minority in suggesting that gravel is not only OK for Cory's but might actually be better. However, I can only report what I've experienced and my other thoughts and observations do I think have at least some logic behind them!

 

[Magnum Man]

I use a lot of natural river gravel, of smaller size… rounded edges, and smaller than most gravel, but too big to be called sand… that has become one of my favorite substrates…

But the bulk of my Cory’s are in a black sand bottom tank, with large river rock on one end… the Cory’s do graze in and around the big rocks, but spend the bulk of their time in the sand… but I honestly never see them nose diving in the sand

I have some old Cory’s, that were in a larger stone river gravel than I’m using lately, and they lost their barbels… after moving them in this newer tank with some new Cory’s, they grew their barbels back… I figured there may have been pockets of bacteria in the other tank, in areas around plants, and hard scape, that I couldn’t regularly vacuum, that may have been the cause for their loss of barbels???

 

[Colin_T]

Maybe the fish are captive bred and have never encountered sand before