Bacteria, to my knowlege, isn't visible to the naked eye. However, with large colonies concentrated in a small space, i.e. bio-wheel, the effective darkening can be seen.
That said, the stuff you're seeing isn't Bacteria. It is some form of algea. I'm no expert on algea by any means, but have found that if you leave the lights off for a couple of days, the algea is gone. That is if you don't have any natural light penetrating the tank. You may want to be careful about this if you have live plants, they need light as well. However, most hardy live plants will tollerate a couple of days without "sun."
Also, there is the root problem to be concerned with. The reason you have algea is because 1. you don't have live plants to use up the nutrients in the water, or 2. the algea is out competing the plants.
If you don't have live plants, I would suggest getting some, unless your inhabitants make it ineffective, like African and large South American cichlids. These plants draw nutrients from the water, and compete with the algea for them. There are several hardy plants available. I suggest some form of Anubias. A. nana if you have a small tank, A. bateri (sp?) or A. coffefolia, or something like a sword if you have a large tank.
If you do have plants, you need to make sure that ALL of the requirments are met in equall amounts. Making up for low light with fertilizers only worsen the situation. The main requirements are nutrients (left over food, fish poo, fertilizers,...), light (for the photosythesis process), and CO2 (for the photosythesis process).
The theory is, if all requirements are met sufficiently, and in equal amounts, then there "should" be nothing, or little left for the algea to feed on. You may want to consult a person more knowlegable with this for advise. Someone that can actually see the tank, or knows what is available to you, would be great help with this. And who better than a trusted, knowlegable LFS? If none are available, or they just don't know a fish from a cat, then I would talk to some of the other people in this forum with greater detail about your tank.
I know its long, and I apologize. But, I have been thinking about this a lot lately.
Here's my theory. If you, an evolved higher organism, and a mouse were stuck in a sealed room, and only one grain of rice fell into the room every hour, and only 1/2 cup of water was slid through the door every day, and only so much air was allowed into the room, Who would thrive, or even survive?
However, if a steak was slid through three times a day, with potatoes and a salad, and you could get a gallon of water every day, and you had plenty of air, Who would thrive?
That's kind of how it is in aquariums, the only stuff in there is what you put in there, knowingly or not. So, who do you want to make thrive?
