It is not recommended to keep your sourdough starter in a hot water tank. While the hot water tank may provide warmth, it poses potential risks and is not a suitable environment for your starter. Hot water tanks are designed for heating water and are not meant to accommodate living organisms like sourdough starters. Placing your starter in a hot water tank can expose it to excessive heat, fluctuations, and harmful chemicals. It's best to explore other methods, such as using a warm spot in your kitchen, insulation, or specialized equipment like proofing boxes or yogurt makers, to ensure a safe and controlled warm environment for your sourdough starter.