For assessing the particulate fouling of water, the modified fouling index (MFI_0.45) is a superior test to the silt density index (SDI). There is a need to compare both tests in terms of sensitivity, how they are affected by the filter material and the type of support plate and also illustrate their use for monitoring of seawater quality over time. In this work, we studied seven different filter holders with different filter support plates and three different 0.45 μm filter materials, and we applied the tests for monitoring of North Sea water quality. The results illustrated that the type of support plate of the filter holder greatly influences the measured MFI_0.45 values and thus, a correction for the effective membrane area may be needed when carrying out an MFI_0.45 test. An attempt to normalize differences in MFI_0.45 due to filter material with a Formazin solution was tested but proven not successful. When monitoring the seawater, the MFI_0.45 was much more sensitive than SDI to water quality variations in particular during algal growth. As the SDI and MFI_0.45 tests can be measured with help of the same equipment, more alignment in the ASTM protocols for both methods is recommended.