Burgermeister: There is no regulation of microbial water-treatment products since these products are not (technically) marketed as algaecides. Actually, if an unregistered microbial product makes an "algae control claim", it is somewhat in violation of FIFRA (Federal Insecticide, Fungicide & Rodenticide Act), which requires that any product making pesticidal claims must be registered as a pesticide with the EPA. Even biological agents (ex. BT) are required to be registered with the EPA if they're marketed as pesticides. Oddly enough, you can probably check the anti-bacterial hand-soap container sitting near your bathroom sink, and find an EPA registration number on its label.

On the microbial-count issue: Saying that a product contains "billions of bacteria" is a gross oversimplification of quality-inference. My best analogy to this type of vague claim would be something along the line of taking a whole dog-pound full of dogs into the field to hunt quail. Maybe, by pure luck, you'd have one stray dog that might be of use - but its innate or trained abilities would be severely hampered by the other mongrels that came along for the ride.

I think the primary challenge of manufacturing a quality microbial agent involves isolating the desired strains and then producing them in high-concentrations while excluding the "mongrels" from the blend. Also, the method and media in which the microbes are packaged have a great deal to do with their viability and shelf-life. I have not heard glowing reports of liquid-media products, possibly due to limited shelf-life.

Final comment for now: I would not necessarily associate a high-price tag with "quality" in the microbial-agent market. I feel that some marketers know that they won't achieve repeat business (due to poor performance). So, they charge as much as possible for the user's first experience (I'll leave out the obvious analogy for that observation).