Argos Play Sand

Play sand can be good in a heavily planted tank, where it’s full of roots, but it’s very fine and can be a nightmare without plants. It can easily compact and become anaerobic. The commercial aquarium sands are a safer bet.

If you can stir it regularly you can overcome this.

I've only used aquarium sand in my tanks as I tend to have a preference for the darker colours.
I have some species of corydoras in almost every tank, and I enjoy watching them rummage through the sand. They can occasionally be so vigorous in their rummaging that they move sand, plants & decor. Still very cute though. 

I have read that corydoras only like light colours. Sources bring a practical fishkeeping article and a group on Facebook. Do you find their colour changes depending on colour of substrate?

Didn't say that a dark substrate was essential. I posted on a corydoras FB group and asked the question about substrate, and was told to avoid dark substrate.

If it’s of any use to you @barneyadi I have Pygmy Corydoras and I use a finer Aquarium Soil which dark brown/black in colour. They are able to rummage around in it and show no signs of discomfort or ill health.

Ok, this is reply I got about using light sand.

Not really, others colours like black are called sand but under magnification it is sharp and nasty obviously because it is for blasting. Aquarium sand has rounded grains. Remember Cory put their faces in it and blow it through their gills.

So with this in mind, as long as I use fine aquarium sand or soil, the colour might not be as important as I thought.

In the past I bought a black sand, can't remember which brand, and when I got it home I realised it was not suitable. It felt sharp & gritty rather than sandy, but was the only really black substrate available at the time, and it ended up not being used. A few people on the axolotl forum also noted red patches on their axies, and it turned out they'd bought this "sand" as well..

I will note that we've all met people who have done things not quite in accordance with the rules, but their fish have been fine anyway. Perhaps opinions have changed, perhaps sand colour might be more or less important since that PFK article was written, perhaps my fish have just got used to what they have in their tanks, perhaps pants & decor are just as important as sand colour. Who knows. These things are probably quite complicated, and perhaps it's more a case of keeping the corydoras on something soft enough not to damage their barbels, provide enough decor for them to feel safe, and keep them in large enough numbers for them to feel comfortable. I'm no expert. 

The typo is staying as I now have a vision of my corys wearing pants. 

Thanks everyone. I think I will get something like this stuff.

https://www.amazon.co.uk/gp/product/B00G37698Q/ref=ox_sc_act_title_1?smid=A3P5ROKL5A1OLE&psc=1

Seems to be good enough.

Looks like very fine grain in the picture, and description says it is suitable for bottom feeders & barbed fish, so I'm sure it will be great.
Just check how it feels, and make sure it's rinsed thoroughly before it goes into the tank.
Please keep us updated on progress. 

Strangely I have found something else called Unipac silver sand which has granules 0.3 to 0.4mm which sounds ok, and is cheaper.

Corys are found all over tropical and subtropical South America, living on all different kinds of substrate from sand to leaf litter to mud. They have adapted their colours and patterns to be camouflaged against the substrate. All species are happiest on a substrate the same colour as they are.