A fuel system for an engine of a marine vessel, the fuel system comprising: a settling tank for cleaning fuel for the engine; a service tank for storing fuel for the engine; and a filtration system comprising a first filter through which fuel flowing from the settling tank to the service tank passes, wherein the filtration system is configured to be operable in a first mode in which the first filter removes contaminants from fuel flowing from the settling tank to the service tank, and in a second mode in which fuel flows from the service tank through the first filter to dislodge the contaminants from the first filter, and wherein the filtration system comprises a second filter for removing the dislodged contaminants from fuel that has passed through the first filter when the filtration system is operating in the second mode; wherein the fuel system comprises a second filter pump for pumping fuel through the second filter; wherein the filtration system comprises a first circulation valve and a second circulation valve, which are joined by a circulation path; wherein the first circulation valve is located between the first filter and the second filter; wherein the second circulation valve is located between the second filter and the settling tank; wherein the first and second circulation valves (228, 229) are controllable to direct fuel pumped by the second filter pump in a circuit comprising the first and second circulation valves (228, 229), the circulation path, the second filter pump, and the second filter, so that the second filter pump is able to cause fuel to be passed through the second filter and back to the second filter pump plural times; and wherein the circuit is isolated from the first filter by the first and second circulation valves (228, 229), so that the filtration system is operable in the first mode while the second filter pump is causing fuel to be passed through the second filter and back to the second filter pump plural times. a settling tank for cleaning fuel for the engine; a service tank for storing fuel for the engine; and a filtration system comprising a first filter through which fuel flowing from the settling tank to the service tank passes, wherein the filtration system is configured to be operable in a first mode in which the first filter removes contaminants from fuel flowing from the settling tank to the service tank, and in a second mode in which fuel flows from the service tank through the first filter to dislodge the contaminants from the first filter, and wherein the filtration system comprises a second filter for removing the dislodged contaminants from fuel that has passed through the first filter when the filtration system is operating in the second mode; wherein the fuel system comprises a second filter pump for pumping fuel through the second filter; wherein the filtration system comprises a first circulation valve and a second circulation valve, which are joined by a circulation path; wherein the first circulation valve is located between the first filter and the second filter; wherein the second circulation valve is located between the second filter and the settling tank; wherein the first and second circulation valves (228, 229) are controllable to direct fuel pumped by the second filter pump in a circuit comprising the first and second circulation valves (228, 229), the circulation path, the second filter pump, and the second filter, so that the second filter pump is able to cause fuel to be passed through the second filter and back to the second filter pump plural times; and wherein the circuit is isolated from the first filter by the first and second circulation valves (228, 229), so that the filtration system is operable in the first mode while the second filter pump is causing fuel to be passed through the second filter and back to the second filter pump plural times.