Fountain question

If this has been addressed before, please forgive me. I couldn't find it if it has.

We recently bought two fountains, and my husband and I are at odds as to whether we can leave them plugged in all the time or if they have to be turned off after a period of time.

I want to leave them plugged in, and he keeps unplugging them. ^_^

Does anyone know if they can be left plugged in and running, or do we have to turn them off after so many hours?

Thanks!