I am a lifelong industrial technician/carpenter, last decade or so I have been working in transit. I currently manage an operation that dispenses about 25,000 gallons per night. 11,000 from the tank farm of 120,000 diesel gallon capacity that is 75 feet from my office. 6x20K gallon diesel tanks and a 20K gallon unleaded tank.
USLD - needed but it is a bane, depending on how fresh it is will often tell how much water will perk out of it. There are several methods for removing the sulphur but one that is most effective and gaining the most ground is adding oxygen then using water to dissolve the sulphur from the ionic solution. This method is preferable because you wind up with byproducts that can be used in other industries as opposed to the older methods which left large unusable waste streams.
So, what does this mean? The centrifuge is going to leave some suspended water, the perking process for evaporation is often lengthened or shortened depending on the volume of orders for the product. Your diesel is getting delivered and dispensed with some water. Algae, fungus, and bacteria become an issue eventually. As tolerances get tighter, injectors get more and more high tech, finer and finer filters are needed to protect them.
Having said all that I use a ton of Power Service products. They just work. I can easily run into the hundreds of thousands of dollars in roadcalls and downtime if the tanks are not kept clean. Fuel cleanliness is not a huge issue because of the turnover we have in inventory. This also goes for the tanks in the vehicles. I use Bio-Kleen biocide and Clear-Diesel fuel and tank cleaner together. One kills and the other dissolves and disperses. They have some cetane boosters and cold weather additives but I have not had any use for them. Our fleets are designed for USLD and our winter lasts a week, sometimes two if we are lucky enough to have it go that long.
So, if you have a modern diesel, use what you need, not what someone is telling you that you need. If you have an older diesel use the products that you need to ammend the fuel.
There are affordable test kits to test for algae/fungus/bacteria in diesel. I tend to use the culture type, I feel like I get a better view of exactly what and how fast things are growing. That way I can treat accordingly. You will need something to grab bottom samples. Bacon bombs are good for this if you have a clear drop to the bottom of the tank.
There are other products out there, they may be effective, this was the first option while spending tons of money that actually worked for us. Mechanical cleaning of the tanks and fuel polishing can run us better than 30K$ per year, these two chemicals give us a better level of performance, reduced roadcalls and downtime, cleaner tanks for about $9,000 per year. One of the few times saving money saved money in all aspects.
Last thing, getting the water off the bottom of the tank and keeping it off the bottom of the tank will do you better than more treatment. The bugs only grow on the split between water/fuel and where the water hits air. They can grow down sump tubes and the walls of the tanks due to following the water with the condensation effect. Thus keeping your tank full is really another of the best things you can do. Fill up after use and not before use, fill up more frequently. Also, your bugs are not growing while frozen, your biocide will not do it's job while frozen. Treating in freezing temps is a waste of money.
Disclaimer: I do not work for these people, I just give them money for a product that works. The two products I have mentioned do work and are worth the cost in the Central Texas area. YMMV (but I don't think so)
Pardon the grammatical errors, I am fluent in Redneck, English is my second language.