Aquarium sand vs gravel: what is the difference between sand and gravel? Other than the obvious difference in size, sand is most commonly used for saltwater aquariums, while fish tank gravel is used for freshwater fish tanks. Most reef sand raises pH, which makes it undesirable for most freshwater aquariums. There are some exceptions, like freshwater cichlids that enjoy higher pH. Fish gravel works great for freshwater because it doesn’t affect pH and is easy to clean with an aquarium gravel siphon.