I think a UK gallon is a US gallon and a half so this monster tank is 5764.8 US gallons. This beast is big and respectable by my standards. What I like about this is the guy told of the cost of building it so this makes it possible to plan out the costs of building one of these in the US where matirals and space to build one of this size should be cheaper.
Currently the US does not have the world's biggest known privetly owned fish tank right now the biggest one I know of is a reported 20,000 gallon salt water reef tank in IL in a five million dollar home somewhere.