The investigated fish pass is a part of construction of а very low-head hydrostatic pressure machine (HPM) located on existing weir in the upper Iskar River. Monitoring of the hydraulic features of the fish pass during the different hydraulic conditions was performed. Diversity and abundance of the local fish fauna was carried out with the aim to verified target fish species. A major limiting factor for using of the fish pass during flood periods was high water velocity (both inside the fish pass and at the fish pass downstream entrance), while during the low water discharge periods it was the insufficient water depth in some pools and the significant differences between water levels in the adjacent pools. The difference between the water levels at the entrance and exit of the fish pass reduces the chances for access of small-sized fish species. For improving fish pass functionality, some measures were developed. Several re-construction solutions were proposed in attempt to help solving the identified problems.