That's not much weight, 500 UK gallons of water = 5000 lbs
Taking a flow rate of 0.10litre/sec for a shower the Emirates A 380's have I believe 80 First and Business class seats, so allow 5 minutes in the shower =
Flow 0.10x60 = 6 litres x 5 minutes = 30 litres times this by 80 and you end up with 2400 litres of water or 2.4 cubic metres, or for the mass 2.4 tonnes