If you scroll down to ornaments, plants, and substrate the site has this about substrate:
So what should I use for my freshwater tank? All species are 4in and under, mostly livebearers, and not excavators.You have to bear in mind that there are advantages and drawbacks of using sand or gravel. Let's start off with gravel. What are the advantages? Well, apart from disguising a lot of the dirt, waste also tends to work its way under the gravel so that tank doesn't look so dirty. You can remove the waste with a gravel vac. The disadvantage of this is that waste can get missed if the gravel isn't cleaned properly. This could have a detrimental effect on the water quality. Now let's look at sand. The advantage of using sand is that waste doesn't get trapped underneath, it sits on top. This means that the filtration tends to remove it more efficiently than if you're using gravel. If you are using a light coloured sand, you can see the waste a lot easier when you are cleaning. The disadvantage of using sand is that because the waste sits on top and doesn't go underneath, the tank can look a little dirtier than if you were to use gravel. So I suppose it's six of one and half a dozen of the other. Personally I prefer sand. I'm more confident that the tank is cleaner because the filtration removes dirt quicker.
One important thing that you have to bear in mind when using sand is dangerous gas pockets can build up so you should try and move it around a little bit when cleaning. You do have to be careful because the finer it is, the easier it is to suck up. It is very important to mention that our substrate will be a haven for beneficial bacteria, just like what is inside your filtration. For this reason, you don't want to clean the substrate too much. What you are trying to achieve is removing waste, that's all. A little bit of dirt is good, you don't want a spotless fish tank.