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If the "volcanic soil" is actually a type of soil (as in dirt), it must be covered. Not only does this help prevent fish like corydoras catfish from digging in it, it also may be sharp and harmful to substrate fish.
You will also need to have some sort of permanent barrier if you intend having the soil only in one area. Water movement will mix different substrates, aided by fish activity, and before long it could be quite a mess.
To bee honest, I would not use any plant substrate in tanks with fish, especially cories, but with care it can be done.
Carpet plants are not easy. The light is the most important factor, depending uon the species of plants. You also want open areas of sand, lots of it, for the cories to dig into.