There are many different types of ready-made systems on sale, which are designed to simply and easily connect to your pond.
These are usually sold with claims of what they are suitable for. In general you get what you pay for. Do not expect the same performance from the budget end of the market as you would from the more expensive systems. But also be careful of exaggerated claims or sales talk. Be cautious of the claim “This Wizzy Gizmo 10,000 is suitable for a ten thousand litre pond….”. This only means that it will work well on an average pond up to ten thousand litres. There are a whole series of other factors to take into account. How many fish will you eventually have, what will be the ultimate size to which they will grow and especially what will they eat, are the most important factors. Remember that a size 1 (150mm – 250mm) koi has the ambition to grow to a size 5 (550mm – 650mm) or larger in a few years. So will all its mates! As will the extra fish that you bought in the meantime. Clearly, unless you are already experienced enough to be confident that you know all the pros and cons about filters you need advice.
While Koi dealers are primarily salesmen, they will not be in business long if they give poor advice in order to maximise their sales. But different dealers have different viewpoints and as a result may appear to give conflicting advice.
I cannot stress too strongly that you should join a club. By meeting with other Koi keepers you will get first hand experience of how the “Wizzy Gizmo 10,000” actually performs in practice. Is it difficult to maintain? Does it need to be painstakingly stripped down and cleaned every week or so? How much does it cost to run? There are no right or wrong solutions in filtration, just different ways of tackling the same problem. Koi keeping is neither exclusively an art, nor a science, it is a combination of both.