In-tank filters are a special type of low pressure return line filters. In-tank filters are mounted directly to the reservoir tank top and have an accessible head that is located outside of the reservoir while the body of the housing is located inside the reservoir. The exposed cover allows the element to be easily replaced as needed. The filter housing may be equipped with diffusers to ensure that the returning oil energy is gradually dissipated within the reservoir fluid to minimize the potential for aerating or foaming of the oil. In addition, the diffuser helps direct the fluid outward against the walls of the reservoir to aid in the heat transfer capability of the reservoir.