Aquatic Microbiology studies the ecology of micro-organisms in aquatic environments. Aquatic micro-organisms include viruses, bacteria, phytoplankton, fungi, and zooplankton.It can be molecular based all the way up to larger scale ecosystem levels.

Here is a good slide review of the field and it' is very much worthwhile to go through it.

Folks always carry on about how they want to learn, well:

Introduction

It's easy to read and work through.

Give it a read, most hobbyists know very little about things that play such a large role in our tanks.

Regards,
Tom Barr