I'm not sure 'fishes' is valid (British) English either TBH, maybe it's a pointer to a more general rift between UK and US language use. The OED does list 'fishes', but I've never heard it used except in jest or by people trying to be cute. Lolcat speak :)
We would say 'The sands of the sahara' and 'different types of sand' or 'different sands' sound equally valid.
See also formal versus notional agreement. British English tends to favor notional, prime example being sports commentary: "Liverpool are playing well" versus "Boston is playing well". Wikipedia highlights some other good examples: "The staff is busy" versus "The staff are busy", "The Rolling Stones are a classic band" versus "The Rolling Stones is a classic band".
The differences sound stilted if you're not used to them. Except, of course, when they're poetic: "Oliver's Army are on their way/Oliver's Army is here to stay".
