Can I use volcanic substrate / soil on its own or do I need to have gravel or sand over it?

The use of volcanic substrate within aquariums is becoming more and more popular alongside Aquascaping. This is due to how nutrient rich volcanic substrate is.
The most common question surrounding this is whether you require the use of gravel or sand on top of the substrate.

You can use volcanic substrate in an aquarium as the sole substrate with no outwardly negative effects. The only downside is the fact that a lot of the nutrients can actually escape into the water column if you have a fast, strong flow. Waste products and excess food can also find themselves trapped between the relatively large gaps in the substrate.

Having it as the sole substrate provides the benefit of allowing plants which depend on more absorption from their leaves rather than their roots.

However, people commonly use more attractive gravels or sand to go on-top of the volcanic substrate. This looks more aesthetically pleasing as well as trapping the nutrient in a “soil” bed, allowing easy access for root plants as well as dosing fertiliser and macro nutrients into the water column to better cater towards all plant types.

You will find, like with soil, some finer particles of this substrate will float. We therefore would recommend that you put a thin layer of either sand or fine gravel over the top of the substrate.