Common indicators that a microbial issue is occurring typically start with slowed fuel flow caused by filter plugging.
Filters that are clogged often become coated with contain dark slimy substances in the filter media.
Many forms of microbial growth also cause damaging corrosion that can cause a host of other problems once it has started. By far, the most common shared element between all types of microbial growth is the presence of a water bottom in the tank where the growth is occurring.
No matter how it shows up, microbial growth can be very frustrating to end-users.