Kay,
Do NOT add fenbendazole directly to the tank water, it should only ever be fed in food.
You can however add flubendazole (simialr name and product) to the water and it has proved effective for camallanus worms. I note you are in the UK which means you can get Flubenol15 (flubendazole) this is a med I have used many times on my discus, though not for camallanus worms.