Connect 25 mm pond hosetail to 3/4" BSP parallel (G3/4 / BSPP)
How to join 25 mm barb (to 3/4" or 1" BSP) and G3/4, and what stops it leaking.
These connect directly. 25 mm pond hosetail mates with 3/4" BSP parallel (G3/4 / BSPP) without an adapter.
A 25 mm pond hosetail with a 3/4 BSP thread screws straight onto a 3/4 BSP pump or filter port, sealing on the thread with PTFE tape, then the 25 mm hose pushes onto the barb and is held with a Jubilee clip. This is a direct, standard pond join.
The join has two seals: the BSP thread at the pump and the clip on the barb. A weep at the hose end is almost always a missing or loose Jubilee clip, not the thread.
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Sources: Pond hosetail / BSP barb size range, Discount Leisure Products · Male BSP barbed pond hosetail connector sizes, Aquatix-2u · ISO 228-1 (G / BSPP parallel pipe threads) · BSP thread dimensions chart (ISO 228 / ISO 7), AmesWeb · Washing-machine / dishwasher inlet 3/4 BSP connector data