Original pond with undergravel filter Installing an External Filter Box Running in the External Filter Removing the gravels, uncovering the messing wiring and pipes Completed modified pipings A "new" pond with good filtration and water circulation A new home for the happy kois Cystal Clear water with no ammonia and nitrite

I am very happy at the beginning of the year 2011 to have the pleasure to help a hobbyist to overhaul his one year old koi pond.

I think it would be good to share this experience and knowledge here so that all can benefit from what we went through.

The original filter system

I was reading the posts in arofanatics forum end of last year and came across a post: Newbie need advice on water feature turn koi pond.

There was a discussion going on regarding undergravel filtration in this threat and I shared some of my opinions on this type of filtration system.

Undergravel filter. How does it work?

To set up an undergravel filter, a layer of plates is placed on the floor of the tank or pond. This is then overlayed with a layer of wool to support the gravels so that they do not drop to the bottom of the pond. And finally, a thick layer of gravels is layed on top of the wool to act as a substrate.

A pump is normally placed at the corner of the pond with direct access to the bottom plates.
Water is drawn from the bottom of the plates and pump out to circulate the pond.
When water is drawn from the bottom, it creates a low pressure at the bottom thus pulling water from above the gravels towards the bottom.

This appears to be a good way of filtering water as it produce crystal clear water that requires minimum maintenance. Owners do not need to clean the filter as the waste of the fish would be suck into the gravels and consumed by the beneficial bacteria.

Sounds too good to be true and it is. The catch is that it is only suitable for fishes will very low bio loads or simply fishes that do not produce a lot of waste. On the other hand, we don't even need a filter if we are keeping fishes that produce little waste, regular water change would suffice.

 

Undergravel filter for Koi

We know that kois produce tons of waste, no wonder some call them water pigs. ;-)

The bio loads that produced by kois will no doubt overwhelm the undergravel filtration system as a matter of time. The problem is that we don't know when, so it's like a time bomb which may explode anytime.

In other filtration systems, the waste will get flush off regularly during filter maintenance. However, in undergravel system, there is no effective way to clear off the waste trapped in between the gravel without disturbing the bio filtration. The organic waste will slowly build up, becomes anaerobic which encourages the colonization of harmful bacteria. Now the good and bad bacteria are all mixed together and fighting with each other for survival. Overtime, the harmful bacteria will overcome the beneficial bacteria because they trive in dirty environment.

Another short fall of undergravel system is that the filtration is uneven on different surface of the gravel. We know that water will find the path where there is lowest resistance. As the Dirts are built up unevenly, some parts of the surface will get highly water flow rate than others. Those parts with lower flow rate will be more lacking in oxygen and result in the build up of harmful bacteria. Therefore the efficiency of the filter will decrease over time.

So my advice to him is to revamp the whole filter system.

External Filter Box

After studying the design of his pond, two options were opened to us.

The first option is to partition his existing pond and create a space for built-in filter. The second option is to get an external filter box.

After some discussions, we decided to go for the second option because of the following reasons. Firstly it is easier to set-up. Secondly, it would not 'eat' into the pond area and provide more space for the kois to swim around. Thirdly, the filter area would be bigger and provide more effective filtration. Lastly, external filter box allows for easy maintenance as it comes with back flushing bottom drains that can easily discharge the waste in the filter compartments.

 

The Hard Works begins ...

The first step is to install the external filter box. The inlet and outlet holes were customised according to our needs. So we took soome measurements such as height of the pond, the thickness of the water feature wall so that the holes can be aligned to these dimensions.

The next step is to let the new filter run in for about 1 week before we start to remove the gravels (Ideally should run-in for at least 2 weeks but we don't have the patience ;p). This is to allow more time for the new filter to mature and populate with beneficial bacteria which consumes ammonia and nitrite that are lethal to the kois.

However before one week of run-in, the owner had already removed all gravels from top layer. Fortunately, the filter was able to perform its job as he controlled the feeding. (Ammonia and Nitrite spike is very common at this stage, extra care should be given at this stage) 

The next step is to remove the extra wiring for underwater lights and pipes from the old undergravel system. The orignal system uses 4 pumps for filtration, 1 x Nova 200 for sucking the water under the gravel and circulate it to the far end as well as 2 pipes running across the center of the pond; 1 x Atman 107 to drive the water to 2 featured waterfalls, 1 x Atman 106 to drive the water to the 3rd featured waterfall and 1 x Atman 106 to drive the water to a 50kg water feature pot.

The pumps were reconfigured as follows. The Nova 200 is used to deliver the water from the outer-right corner of the pond. The other 3 Atman pumps were placed at the outlet of the filter (Inner-right corner) to deliver the filtered water to 1) the featured waterfall, 2) the far-end, left-outer conrner and 3) directly beow the filter outlet. This arrangement creates a water flow in an anti-clockwise direction which draw the water towards the Nova 200 pump thus providing a good water circulation.

 

Result after 2 weeks ...

The owner was very satisfied just after one week of using the new filter set-up that he bought 2 better quality kois (1 Kindai Show and 1 Kohaku, in the last picture on top and below the Shiro Utsuri) and he is even more happy to have gain 10cm depth of pond volume to house his kois.

As the filter was not fully run-in, the kois were flushing after the new kois were introduced. This was overcome by adding some salt to mitigate the effect of Nitrite spike.

It's just over 2 weeks now as I wrote this article. The filter is more stabilised now, there was no Ammonia and Nitrite measured and the kois are happy in their new environment.

I am happy to share this experience so that those who want to start a new pond or modify their existing filter system can have the confidence to do so. Remember, keeping koi is about keep good water. Keeping good water is to have a good filtration system and maintain it properly.

Happy Koi-ing !!