It’s none of my business, but what I would do rather then go to the trouble and expense of putting in another tank is to educate your daughters in taking short quick showers, keeping the flow at a minimum ( not turning taps on full.) Plus you could put in low flow shower heads. Hope this helps, without offending you!
novaman72
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Life is not measured by the number of breaths we take,
but by the moments that take our breath away