Don't be afraid of the microscopic world. It isn't out to get you.
We all eat and breath mold all day; yes, that bread or cheese you bought has mold all over it already. Most, if not the majority of mold isn't dangerous. Breathing large amounts of it can cause problems if your sensitive, but eating it (ingesting), well, maybe if it's in the hallucinogenic family of fungus things could be interesting.
I doubt mold would grow in a fw tank, there's nothing in there for it to eat. Algae also won't grow, there's no sunlight.
Bacteria already in the water and air are reproducing in the tank, like it or not; that's really what the sanitizing process is meant to take care of. Unless you're filling it from non-potable sources, the only bacteria in there are ones you've already been exposed to and if they haven't killed you yet, what difference will it make.
I'm more worried about getting side-swiped by someone on the expressway, or a freight truck running a red light.