Tip: Saxham Reservoir is the hardest place to catch fishes, but all fishes there are available 24 hours, so you have all the time in the world. Each of the Carp fishes there, with exception Lucian Carp, react only to one certain lure and you have to hit them directly on the head with it, probably multiple times until they start reacting. They are not hard, since they have almost no strenght, but a lot of stamina.
Use this video, to see how the fish looks and find it in the water and then just keep throwing the lure, until it bites.
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