Probably not, assuming the sand isn't much more than 1 or 2 cm deep. Corydoras will burrow right into the sand, and you'll see them spewing it out through their gills. It's fun to see.
Oh, I have two cories, so I do not have to worry about anearobic(sp) gases?
Just to clarify the anaerobic decay and gases bit. Assuming you don't have an undergravel filter or an undertank / in-gravel heater, oxygen only gets into the substrate by diffusion. In a gravel substrate, the gravel particles are so big that the diffusion of oyxgen is rapid and effective, and even a gravel substrate 10 cm (about 4 inches) deep will be perfectly well oyxgenated.
In a sand substrate, the size of the particles means that they are much more closely packed, so there is less space for the oyxgen to diffuse through. The result is that oyxgen concentration drops off rapidly the deeper into a sandy substrate you go. In practical terms, below about 5 cm depth, and certainly before 10 cm depth, the oyxgen concentration will likely be too low for oxygen-consuming bacteria to survive. This is called the "anoxic layer". This means that any decay of organic materials there will be
anaerobic (just like yeast brewing beer or wine). In the process, whereas yeast produce carbon dioxide and alcohol, these bacteria will produce, among other things, hydrogen sulphide.
In high concentrations, hydrogen sulphide has a nasty, bad-eggs smell, and is toxic to most fish.
Now, there are two addition factors to consider. Firstly, plants aerate the substrate. Aquatic plants have evolved to have their roots in sediments with low oxygen concentrations (i.e., mud) and so absolutely love to be planted in sand. Why? Because they can much more easily absorb minerals in low-oxygen sediments than high-oxygen ones. That is why plants grow poorly in tanks with undergravel filters, which by their very action oxygenate the substrate. Anyway, the plant roots carry oxygen down so that the cells in the roots can 'breathe', and in the process oxygen escapes into sand stopping it becoming completely devoid of oxygen.
The second factor is snails and fish. Burrowing Malayan livebearing snails in particular will dig into the sand and, just like earthworms on land, they allow oxygen to diffuse down their burrows and freshen up the sand deeper down.
So, if you use plants and the right kind of snails, and perhaps a few digging fish as well, then sandy substrates are entirely safe. They may also have other benefits beyond improving plant growth. Nitrate-removing bacteria will live in the anoxic layer, and these break down harmful nitrate into harmless nitrogen.
Hydrogen sulphide is really the main problem, and for this to be produced in appreciable and dangerous amounts, you need a lump of organic material to get buried in the sand quite deeply (5+ cm) and left there for at least a few days. Simple aquarium hygiene should prevent that happening, as will scavenging fish, Malayan livebearing snails, and the fact that stuff tends to sit on top of the sand rather than sink into it.
There's a lot of paranoia about sand and anaerobic decay, but it's important to realise that such conditions are normal in the wild and perfectly manageable in aquaria.
Cheers,
Neale
