Dennis,
Microbial growth (commonly called algae) in a Beaver fuel system is not common unless you live/store it in a very humid area, and you do not keep the fuel tank completely full when not in use. However, if you would like to be proactive and use a biocide as a preventative, you can buy one at most Walmart stores, especially those in an area with a nearby marine industry. Be sure to do your research, and do not confuse a standard diesel additive with a biocide. While a biocide is a diesel additive, not all diesel additives are a biocide.
Gerald