Question

would there be any bacteria in an aquarium

Answer 1

Yes bacteria do exist in water bodies
They are in aquarium as well
One of the most important things an aquarium hobbyist can do to maintain water quality is seed their aquarium with good bacteria from an established, stable and healthy tank.
Good bacteria can come from a handful of populated gravel or substrate, or a used filter pad that’s been rinsed in tank water, a used sponge filter, or even an external filter box itself. Dropping a piece of used filter pad into a new filter box helps establish a colony of good bacteria in a new tank. Growing and maintaining good bacteria is probably the single most important thing an aquarium hobbyist can do to keep a healthy tank. Protect good bacteria as if it were gold, and so many of the most common issues may never arise.

Answer 2

yes there would be bacteria in an aquarium.there is no such place where bacteria does not exist.even it is present inside our body.This is because these can withstand climatic changes.