the tanks have to have a fill valve and only have static pressure to drain. If you want to do something on the cheap simply get someones lightly used water heater on craigslist, you can find them for $50 in good working order. Put them in line with your (working) hot water tank as the last one. This way they water comes in from the city line at about 58 degrees, warms to room temperature about 68 degrees before going into the heated tank and you save all of those BTU's you would have to burn to add 10 degrees, constantly fills and tops itself off and cycles your water so it doesn't go flat.

you'd save 4170 btu's per tank of water.

1 BTU = 0.000293 KWh

1.22181 kwh per tank saved at about $.14 per kwh which means you'd save the $50 cost of the used tank in about a year (294 days) assuming you only used one tank full a day

Or just buy a plastic tank, stick it in the corner, fill it and cap it. Water doesnt go bad...