KOI AND PLANTS:
There is always going to be controversy about whether a Koi pond should, or should not have Plants and we are often asked 'Why we don't have plants in our Koi ponds ?'
First of all .. Koi are bottom feeders, that means that they feed from the bottom of the pond - scratching around, digging where they can in so created natural ponds, and they are known for eating vegetation of all sorts .. this includes plants .. so Koi and plants do not really mix ... but it has to be said that many Koi keepers have kept plants and Koi together for a great many years without any real problems being created ...
By all means if you wish to have plants with your pond, and we certainly are not going to argue the point that plants, Lilies, Iris's, and other water marginal plants do not enhance the pond ... it does .. and nobody can argue against that fact ...
But there are ways for you to have such plants - providing that the Koi cannot get at them .. If they do then there is no doubt that they will eventually tear them to pieces as fast as they can grow.
So how can you keep the Koi away from the plants ... well firstly decide that the plants that you are wishing to keep are of the marginal variety .. and this can of course include those beautiful, even if short lived, Iris ... and then at the time of the construction you must make provision for these plants.
One way whilst you are constructing your pond is to build a 'shelf' which steps back and which is wide enough to place rocks along the edge of this shelf which will then allow you to fill the back area with soil in which you would place your plants .... These rocks will not only hold the soil back and prevent the water washing it all away and into the pond, but they will also act as a barrier for the plants and make it impossible for your Koi to be able to get at them and totally destroy them ...
We hear a lot about some of the lesser Koi dealers and especially Garden Centres advising customers to place plenty of oxygenating plants into their ponds ... but of course they would say this .. they are selling the plants ... But this advise in our opinion is totally inaccurate as we believe that so-called 'oxygenating plants' have far too many drawbacks in Koi and other fish ponds .. OK yes they do generate oxygen ... but at the time that your Koi and your other pond fish don't need it ... when they are desperately gasping at the surface for air, as they will do, that is the time when they need the oxygen .. but this is also the time that the plants themselves are using up all the oxygen that they can make ....
However: If you have decided to have plants within your Koi Pond - let's look at the facts so far...
1. The appearance of the pond--
As we have said and admitted... a nicely designed and well planted pond is always going to be attractive and create interest for the owners - as opposed to having a completely sterile looking pond without any plants ... But you should also bear in mind that if you are going to have plants, especially deep growing Lilies, then these can cause you problems when it comes to catching up your Koi for inspection or for treatment of a wound etc .... they will run rings round you swimming in and out of the plants where it is almost impossible for your your net to get near them ...
2. Koi eating Plants--
Yes ! there is always going to be a strong possibility that the Koi will eat your plants, and in many cases, as fast as those plants can grow ... so you do have to be careful and selective of where you have the plants within your pond, and certainly, as we have mentioned above, a carefully planned planting area such as building a special wide shelf with Rocks placed in front of the Plants to prevent the Koi from getting at them, is a very worth while consideration ..
In many cases Koi will not attack plants quite as often as they may do if they are fed fresh greens, such as lettuce, or vegetables on a weekly basis - after all please try to bear in mind that Koi are generally and naturally vegetation eaters ..
Another point to bear in mind is Feeding ... One of the mistakes a pond keeper can make, is to simply throw the food into the pond and walk away and not observe just how much the Koi eat at any one time, and some of this excess food can end up amongst the plants. Koi have a great sense of smell and will use any means to get at the food, destroying some of the plants in the process of trying to get at those food particles .. so this is a point that you should bear in mind ... also bear in mind that if you are wishing to have your Koi come to take food from your fingers, then simply throwing food on the surface and walking away will never achieve that ... sit with the koi and be very patient in those early days ...
3. Pond Plant Maintenance--
Pond plants have to be maintained and looked after just like the rest of your garden.
Any dead leaves and dead flowers should be removed to prevent them sinking to the bottom of the pond when they can create more anaerobic sediment with a great possibility of Methane Gas which without doubt is not good for anything within the pond including of course your Koi and other pond fish ..
4. Marginal plants --
Apart from adding additional colour to the pond, plants do help to maintain a better balance and can improve water quality by means of their roots absorbing some of the nitrates. Their main benefit is being able to shade the water surface, as their leaves spread out, providing sometimes much needed shade for our Koi on very warm summer days... (no matter how rare those days are !)
Koi And Plants 
I ended up building a separate chamber off the back of my skimmer which became a veggie filter full of egyptian reed. Much better root structure than norfolk reed (think Jap mat) so more likely to take up nitrate. Don't like the cold though...
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