How do i tell my mom i wnt 2 strt taking a daily shower alone? im scared shell be mad cos of the water bill?

we have a family bath day once a week but we all use the same bath water to conserve cos my mom doesnt want a huge water bill its very very expensive she says. my mom goes first then my dad then my brother then me but its so gross my brother s and s in the water and i have to bathe in it and he does it on purpose. he is 16 and a gross imature little jerk. well anyway i am scared to ask becase last time i mentioned it she started screaming at me and she threw a seramic plate at me which broke and then she blamed me and made me go ask people for 25 cents each until i got enough to pay for a new plate for her since i made her break it