A bit over 20 years ago I built a 1,200 gallon tank in the corner of my basement using ICF blocks. It's main purpose is to store hot water that comes from my wood gasification boiler for use in my radiant in-floor heating system.
Since it was new construction I only had to build 2 sides since the other sides were part of my basement walls. Inside of that tank I have two heat exchangers, one is to collect the heat from my wood boiler and the other one to heat my DHW. 12 years ago we added a ground source heat pump with desuperheater so in the summer I get "free" hot water from that using the same heat exchanger coil. I never did get around to adding the solar thermal components in spite of having dozens of flat plate collectors. Opted to cover the roof and patios in PV.
Here's the thread
where I talk about it. Collecting and moving heat with water is a thing of beauty.
The water in my 20+ year old storage tank is crystal clear and totally sanitary because I regularly get it above 150f.