Hi, replace the shower head with an eco freindly one which will save you at least 6 ltrs a minute and you will still get a great shower due to the head putting air into the water making it feel like you are getting more water at a powerful rate.
you can contact d.moorhouse @ james-hargreaves co uk who i got mine off.
It does work basically your shower head you have now supplys you at 15ltrs a minute (if it is like mine on Jardin 4 los naranjos),
So if you are getting 3 to 4 showers now you should nearly get twice as many.
like the person before says if you put abigger water tank it is going to cost you a lot more in electric never mind the cost of the tank in the 1st place. This shower head will save you water and money in electric.
hope this helps