The best replacement for water tank is tankless.

The best alternatives to water tank are rated as following:
★★★★★  tankless
★★  fresh water tank
★★  on demand
★★  boiler
★  tankless water heater
★  salt water tank
★  instant hot water
★  electric hot water tank
  gas hot water tank
  combi boiler
  electric water tank
  regular hot water tank
  gas water tank

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Speaking about water tank, one probably shouldn't forget about pressure washer.

Other search terms related to water tank and their ratings are:
★★★★★ pressure washer
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Google Trends

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water tank
tankless
fresh water tank