Alum is often recommended by WASH agencies as a pretreatment flocculent to improve filtration in biosand filters (BSFs) for communities using a turbid drinking water source. Floating villages on the Tonle Sap Lake in Cambodia using BSFs encounter severe declines in filtration rates while using alum, resulting in reduced use of the BSF. We tested the effect of rock alum treatment on flow rate and turbidity. The flow rate of all BSFs declined over time, but degradation of flow was more rapid for alum-treated water than untreated water. Rock alum treatments significantly reduced the turbidity of borrow pit source water. Filters switched to untreated river water decreased in turbidity to levels ≤ rock alum-treated river water. Rock alum treatments increased aluminum in source water 4–15 times, but filtration by BSFs decreased levels of aluminum to near 0.05 mg/L. Though rock alum effectively reduces turbidity in source water, we believe it continues its coagulation inside the BSF during pause periods, negatively impacting flow rates.