Which Microsscope?

Hi,

I am wondering whether anybody could help me on the way by suggesting a
suitable microscope, which I would like to use for the purpose of
investigating microbes in aquarium (tropical freshwater) water and biofilms
created in biological filters.

My experience is not great in microbiology, though I have some understanding
of the different types, and shapes of bacteria and protozoa.

I find the topic facinating and hope to be able to identify bacteria and
protozoa feeding on each other (and sometimes on my fish :-).

Thanks,
Sven