If diesel eating critters are the main concern, a biocide is the best way to go. Most of the general additives on the shelf do not have biocide protection built into them. Something like FPPF Killem and some other brands will more than kill all those little bugs. But even then, if one is filling the fuel tank frequently, and buying their fuel from high volume stations, the risk is not that great for getting a critter problem. For stored diesel, a biocide is a good option. And a little goes a long, long way. One 16 oz bottle of FPPF Killem treats 1280 gallons of fuel. And the stuff really works. It is very cost effective compared to dumping bottles and bottles of off the shelf additives in the tanks that might not even address the critter problem effectively. You can usually find FPPF Killem at most commercial truck dealerships and major truck stop chains. One 16 oz bottle of Killem is cheaper than a jug of Power Service or Howes type of fuel additives.