Bottom Drains

Vital part of a modern koi pond.
It is generally accepted that bottom drains are a must and should be fitted to all ponds under construction, those being modified, or alterations such as fitting a new liner.

There are a few cases where it is not possible to connect into the main drains but even in those cases the problem can probably be overcome by use of a soakaway if you do not have nearby neighbours..
If the main drain is above the height of the pond then it is possible to have a discharge chamber, and then you can pump it to the drain, using a pump with a float level fitted.

If your dealer says don't fit bottom drains because they will be blocked up by blanket weed , etc, then do ask around amongst other koi keepers because this is one of the few topics where everyone seems to agree.
There are a large number of bottom drains on the market at various prices, and it is up to the individual to look at as many as possible at the dealers and then make a decision.