There's no shortage of creatures who regard your pond as a feeding station, a bit like a bird table. For species such as herons who have evolved to spend hours hunting brown fishes in murky water, the combination of clear water and colourful fish is a dream come true! Other creatures that pose a threat include cats, foxes, crows and humans to name a few.
Initial pond design can play a factor, such as vertical sides rather than shallows to prevent wading predators but protection falls into two categories - barriers and devices.
In the first category, cover nets are really the beginning and end of the solutions and although not as attractive, they are probably the most effective. If a pergola or other structure is used above the pond, strands of fishing line can be placed at regular intervals to limit the wingspan of any visiting birds.
As you can imagine, more hi-tech solutions are now available including those that spray water or flash and emit scary noises. There's also a range of other solutions including the good old plastic herons, although these are best used by pond keepers with a good aim and a strong throwing arm for best effect.