About: http://webisa.webdatacommons.org/397013709

Property	Value
http://www.w3.org/1999/02/22-rdf-syntax-ns#type	http://www.w3.org/ns/prov#Entity
http://www.w3.org/ns/prov#value	The short answer is no you do not need to cover the bottom of the fish tank with gravel (unless you are using an undergravel filter) but most people think some sort of covering looks better and the fish seem to prefer it.
http://www.w3.org/ns/prov#wasQuotedFrom	petsparade.co.uk