How to Build a Koi Pond - Bottom Drains

Why, What, Where etc - you need bottom drains!

Picture of where to locate a Koi pond bottom drain

OK but why here?

Bottom drains count for the vast majority of the water circulation in a Koi pond - by that I mean that the relative volume of water that they take up from the pond is massive when compared to that of say, your surface skimmer.

The vast majority of solids will sink out to the bottom of the pond in time, where ideally, they will be sucked into the bottom drain and fed to the filter systems where they will be extracted from the Koi pond system.

A rule of thumb from our builder is one bottom drain for the first 15 000l and then one drain for every 10 000 - 15 000 l thereafter. Of course, the size of your pond will have a lot to do with this - as will the flow rate you want through your filter systems. If you turn your pond water over every two hours, and the size is 50 000l you will need 25 000l per hour coming from your bottom drains.

A gravity fed 110 mm bottom drain pipe will flow at approximately 10 000l/hr. This means that our 50 000l pond with 2 bottom drains will turn the pond over once every two and a half hours - not a train smash and within acceptable limits. Three bottom drains will turn the pond over every 1 hour 40 mins, so provided the filtration system can handle 30 000l/hr, this is also an acceptable solution.

Bottom drains - or the pipe work from the drain to the settlement chamber should not be too long. This is because although there is water flow through the pipe work, the heavier solids tend to settle out and can build up in the pipe work itself.

You can see that using thinner pipe although it may have a higher flow rate, will have a tendency to block much more easily. In addition, a 50 mm bottom drain pipe simply will not be able to give you enough flow, under gravity, to feed your filter system adequately unless you have a lot more of them. The additional expense would make the price of the pond horrific.

The only way to make a 50 mm bottom drain work is to suck the water through via a pump. This is terrible design because the pump impeller will just churn the solids in the pond in mush - making it very much harder for the mechanical filter system to strip the solids out of the pond water. The compromise is simply not worth it - USE 110 MM piping on your drains!

Accumulated build up in the pipe work is something that you simply don't want if you can avoid it. Solids accumulating anywhere in a Koi pond are bad news. This is why stand pipes are used - they help to purge the drain pipe work of accumulated solid debris and muck. It is also a good reason to have a settlement chamber...