There is described is a belt cleaning blow-off device having a streamlined inner surface. In one embodiment a fluid outlet nozzle is connectable to a conduit of a blow-off device for cleaning a belt filter. The nozzle comprises a housing having an inner surface and outer surface, wherein the inner surface defines a fluid entry zone to receive fluid into the nozzle and an elongated gap for directing fluid towards the belt. The inner surface is streamlined to facilitate flow of the fluid into the gap. In a second embodiment, the blow-off device comprises a housing connectable to a fluid source and having an inner and outer surface. The housing defines a conduit for transporting the fluid within a channel, wherein the inner surface defines a surface of the channel and an elongated gap for directing the fluid towards the belt. The inner surface is streamlined and typically teardrop-shaped to facilitate flow of the fluid into the gap.