Natural swimming ponds as facilities intended for recreational purposes are characterized by simplicity of solutions and a significant share of natural processes. Ponds are built from commonly available materials found in nature, and their main feature is the use of simple water treatment techniques using appropriate plants and filtering water through porous material. This work is devoted to water filtration processes using two materials, i.e. limestone grit and appropriately prepared rock called Rockfos®. Due to the need to filter water under gravity flow conditions, the aim of the research was to determine the functional relationship of hydraulic losses of the bed made of the selected material depending on the water filtering speed. Knowledge of this function will allow, given the filtering speed for technological reasons, to determine the required height of the water layer above the bed. The research included determining: the equivalent diameter and sphericity of grains in the above-mentioned deposits, the porosity of these deposits in loose and dense packing conditions, and the relationship between pressure losses and filtration speed. Determining these values is the basis for developing a methodology for designing these filters. It has been shown that the multitude of formulas for calculating the equivalent diameters of bed grains „d” and the sphericity „ψ” introduces significant difficulties in modeling pressure losses as a function of filtration speed. The obtained numerical values differ and individual sets of „d·ψ” products do not always reflect the relationship consistent with the measurement results. It was found that the model results closest to the measurement results were obtained using volume diameters and sphericity calculated from the average spatial dimensions of individual grains.