Background: Bacteriophages (phages) have been proposed as an alternative to antibiotics and surface disinfectants for treatment of Flavobacterium psychrophilum biofilms and fish infections in aquaculture settings. The aim of the study was to estimate the minimal phage:host ratio (PHR) required for the control of in vitro biofilm formation and mortalities caused by F. psychrophilum in experimentally infected fish. Materials and Methods: F. psychrophilum cells in different stages of biofilm formation were exposed to the lytic phage FPSV-D22 at different PHRs. Results: Our results show that an initial PHR of 0.01 is sufficient for more than an 80% inhibition of attachment and colonization, and disruption of maturated F. psychrophilum biofilms, whereas greater ratios resulted in almost complete interruption of the different biofilm stages. Interestingly, a similar response was observed in a phage therapy trial with live rainbow trout (Oncorhynchus mykiss), where treatment of F. psychrophilum-infected fish by injection of serial bacteriophage doses resulted in significantly (***p ≤ 0.001) higher survival already at a PHR of 0.02. Conclusions: These results indicate that phages have the potential to be effective for control and treatment of F. psychrophilum infections in fish farms even when applied in concentrations lower than previously expected.