Water Science and Technologies

My house has a tankless water for most of the house. Exceptionally, one floor gets hot water from a tank. I rarely need hot water on that floor so I keep the tank unplugged. But when I need a backup shower (e.g. the tankless gets clogged with limescale) I plugin the tank, let it reach a quite high temp, then shower.

Is this risky? I just heard from someone saying they only unpower their water heater for 1 day at a time because of some specific kind of bacteria. I was assuming whatever bacteria colonizes in 6 months or whatever would be killed off when I fire it up. But I know that some bacteria (which goes after spoiling meat) produces toxins, so even when the bacteria is dead there are dangerous chemicals remaining. Is this the same risk with water heaters?

If it’s unsafe, what do I need to do? Do I have to fill the tank with air between uses? Or can I just run the water for as long as needed to get all new water in the tank before powering it?