The DU Lounge

It seems the predominant brand is Corro-Protec.

Not an advertisement or recommendation, just what I've seen:

https://www.corroprotec.com/

There are less expensive no-name brands too, hopefully not made of toxic mystery metals.

We currently have a tankless water heater. This DID NOT save us money when we had teenagers living in the house! A tankless water heater places no limits on the length of time they can be in the shower.

When I was a kid there were so many people living in our house that my parents had to impose a strict five minute limit on showers to keep the peace. When the hot water ran out taking cold showers was no fun, especially since my dad believed that there was never any good reason to heat a house in Southern California. His dad, who grew up in rural Montana, believed the same.