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Algae, or whatever the sediment is called, is not only in the fuel, but on the tank walls. It is likely organic so an algaecide will kill it and cause what is on the tank walls to slough off. That then becomes the debris clogging fuel filters.
To create that organic mess moisture is required. The key steps however begin with cleaning the tank after material has been added to kill the material and get it to slough from the tank walls. Fuel polishing works. It is a simple process in which a fuel pickup line is inserted into the fuel tank and the fuel drawn into it is run through a series of filters to remove the sediment, and is then returned to the fuel tank. Polishing also removes all moisture.
You can do what Prevost suggests but make sure your credit cards have high limits.
Prevention is the final stage. Keep moisture from the tank. Moisture accumulates as the tank breathes air in. The moisture (humidity) in the air condenses and that small amount of moisture falls to the bottom of the tank. But if the fuel tank is usually full, the amount of air it can contain above the fuel is very little so any condensed moisture will also be very little. If the temperatures where the bus is stored don’t vary, or vary very little then the tank is not breathing in moisture laden air because it is not warming and cooling.
Unless the filters have gotten so clogged debris is bypassing them, it is likely the injectors have another issue. It is not uncommon to have injector issues, and if you do it is strongly suggested to replace all at once.