Diesel tanks have a tendency to attract algae if the tank is allowed to sit for an extended period of time without being filled. This is partially due to the nature of diesel fuel. Algae presents a real problem, as normal diesel engines cannot burn it for fuel, and it can clog the fuel filter, causing hesitation and engine stalling. To remove it, you'll need a commercially available diesel fuel tank cleaner.
Open the petrol tank door and remove the petrol cap.
Pour in 4 ml for every litre (0.64 oz for every gallon) of fuel that you add to your fuel tank.
Run the diesel engine as normal. Diesel fuel engine cleaner needs to mix sufficiently with diesel fuel to dissolve the algae in the tank. For this to happen, you'll need to have the fuel pump operating, and for the pump to operate, the engine needs to be running.
Repeat Steps 1-3 until you have cycled through at least two full tanks of diesel fuel.
Make sure you run your diesel engine regularly while you are cleaning the tank. The tank cleaner will not be effective if the cleaner does not sufficiently mix with the fuel and algae in the tank.