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Building a concrete Koi fish pond - Guide with Pictures

Building a concrete fish pond can seem a large project to undertake, especially for those of us that have no qualifications in building etc. I have built my own concrete fish pond and once the project was underway, I quickly realized that it was not as difficult as I first believed that it would be.

Careful planning and an eye for detail will ensure that this project will run smoothly but I do know from experience that concrete fish ponds do not come cheap, expect to pay more for this project than if you were using a flexible or preformed fish pond liner, what you do finish up with is a fish pond with a lifetime guarantee, no other liners can offer this.

My project was to create a fish pond for Koi fish so their special requirements had to be included in the initial planning, the design was fairly basic as some of the Koi fish ponds that are created have extensive pipe work for bottom drains etc an intricate filtration systems, what I required for my pond was a basic design that would house the Koi fish successfully and the water would still have a high quality without having to resort to an expensive filtration and drainage system.

There are several factors to consider when designing a Koi fish pond as opposed to building a standard fish pond for gold fish etc.

The minimum depth for Koi fish has to be at least 4 feet, as the water temperatures drop over the winter months, the fish will drop to the lower water levels, if these are not shallow enough you run the risk of fish mortalities, in the worst scenario the fish pond will freeze over and the fish get trapped on the ice layer.

Koi fish ponds need to hold a decent gallonage of water; these fish grow large and need a lot of swimming space, for my project I decided to construct a fish pond that was 10 feet in length, 8 feet in width and 4 feet in depth.

There should be no dead spots in the fish pond i.e. the water flow around the fish pond needs to reach all areas, rounding the corners of the fish pond in the construction will prevent any potential dead spots.

Always keep safety in mind, these fish ponds are deep so access for children or pets should be minimized as much as possible unless there are responsible adults with them, prevention from day one saves a tragedy at a later date.

In a large Koi fish pond there is an immense weight of water, the base needs to be constructed from at least 6 inches of concrete preferably reinforced.

Building a Koi pond - ground

This was the ground area that sited my project, unused garden space that was in an ideal situation for the fish pond.

The initial planning purely involves working out how much space you have for your fish pond, allow for extra space as when you start the excavation the slopes will be angled to prevent the soil slipping back in plus slopes will give you access to all areas when you start the brickwork. To help with the safety aspect plus saving me a lot of backache while the excavation was being performed, I decided to build a raised fish pond that was 2 feet below ground and then 2 feet above ground, this saved a lot of time as regards the digging out and looked far better in my view from the aesthetic view point.

For filtration I decided to use a manufactured filter that would be partially buried at the side of the pond, access would be left for maintenance, and this would flow directly back into the fish pond by means of pipe work that would pass through the sides of the fish pond.

The site for the fish pond was only a few yards from the house, this meant that the fish pond could always be in view even if the weather prevented me from going outside for viewing. When planning your site, avoid areas where there are too many trees, falling leaves can rot and foul the water, direct sunlight for long periods can also cause havoc with your fish pond, not only can this cause algal outbreaks but Koi fish do not like direct sunlight.

Due to limited access in the back yard, there was no chance of using a mini digger; I knew that I had to dig out the excavation manually, as my plans involved only going down 2 feet deep this was not such a major problem. One tip to bear in mind though, limit yourself when digging by hand to a certain length of time each day, trying to dig out the site in one day will only result in a bad back for weeks to come.

Once the excavation was complete it was time to concrete the base. As mentioned above the concrete needs to be at least 6 inches deep, below the concrete there also needs to be 3 inches of hard core rubble, to overcome this problem I decided to use heavy duty paving slabs, these are sold at a 3 inch thickness, I simply laid them on a thick layer of sand and staggered the joints with the two layers of paving slabs.

If paving slabs are not available them reinforced concrete must be used, steel mesh is placed on the floor and the concrete is then poured over it. For the internal sides of the fish pond I had already decided to use the cinder blocks (breeze blocks) but went for the solid blocks rather than the blocks with the air gaps in them, using the hollow blocks means that you have to fill the spaces with concrete for added strength, with the solid blocks this is one less task to perform.

The corners of the pond were built up and then the sides laid to fit flush with the corners, The blocks are ideal for novice builders, they are easy to lay and with careful measurements and a decent spirit level it is very easy to get an accurate and level shell for the fish pond. As the sides of the pond rose I backfilled the outside of the excavation with a mix of the soil that was removed during the digging and mixed this with cement, this can save a lot of pennies of using concrete to backfill as well as losing a lot of the excess oil that has been removed initially.

As exterior brickwork is going to be added to the top 2 feet of the build, backfilling of the underground section stopped 2 feet 6 inches from the top of the expected build.

At the top of the build I allowed three holes for the pipe work, two for the outlet of the filter and one at the other end of the pond for the water pump piping to feed the filter. These areas were just below the top of the build as they will be above the water line at all times when the fish pond is complete. I placed 3 inch drainpipe cut to the width of the blocks and sloped gently downwards in the holes and filled the rest with cement to seal. The size of the drainpipe would allow me to feed the filter and water pump pipe work through them as well as keeping the pipe work hidden from view.

To prevent my project from becoming water logged during this time I actually fixed a gazebo over the top of the build, this kept everything dry to allow the cement to go of quicker and still allowed me access to all areas, keeping dry myself.

Once the inner shell was complete things were really starting to take shape, the next task for me to complete was to build the exterior brickwork around the exposed cinder blocks. I deliberately stopped backfilling 6 inches below the required level for the exterior brickwork to allow for foundations for the exterior walls. These were simply created by adding 3 inches of hard core rubble and on top of this 3 inches of concrete was poured over. The foundations were then given 24 hours to harden before any brickwork was attempted.

Rather than attempt to use standard sized bricks for the exterior I shopped around various builders merchants and decided to use the decorative blocks that are available, when priced up they were not too expensive and with my limited experience of bricklaying they were ideal, simple to use and a professional finish could still be achieved. I left a 1 inch gap between the cinder block interior and the outer shell, this was then filled with a dry mix of sand and cement to add strength to the build, once the outer and interior wall were at the same height I finished off the building with coping laid around the top edges.

This was the building complete; the fun part now was to render the inside of the fish pond with mortar which was mixed at 4 parts sharp sand, 1 part cement. The brickwork was coated with PVA before applying the mortar for two reasons; it added another waterproof layer plus provided a good bonding medium between the mortar and the blocks.

To strengthen the mortar I was advised to add fiberglass to the mortar mix, this adds a great amount of durability to the mix making it much more weather and water damage resistant. The fibre glass is sold loose in bags and is not that expensive, it was simply stirred into the mortar mix prior to the mix being applied.

The mortar was first applied to one half of the bottom of the fish pond; this was allowed to harden before the second half was covered with mortar. At this stage I allowed a further 2 days for the base of the fish pond to dry completely before applying the mortar to the sides of the fish pond. Rendering the sides of the fish pond can seem a little tricky to start with but after a little practice it is a very straightforward job, apply the mortar so that the sides of the fish pond are nice and smooth, round the corners of the fish pond to aid the water flow and the mortar is allowed to dry before any further work is carried out on the project.

On its own, the mortar is not water proof so the use of pond paint is required. When I shopped around on the internet I found a company that supplied this paint in 4 colour variations, these were clear, green, blue and black.

I avoided the black colour as it reminded me too much of pond liners and decided to used the green colour instead. Following the instructions I still had to apply two coats of the clear paint followed by two coats of the green colour, the finished provided a good waterproofing and the actual colour was constant through all of the surface area and looked just the job in my view.

All of the paint work was allowed to dry and then the water pump was lowered into the pond, attached to the filter by means of pipe work and the two outlets from the filter were fed through the fixed drain pipes in the side of the fish pond. All that was left to do now was to fill the fish pond and leave it for a few days to make sure that it was in fact holding the water, during this time the filter was switched on so that the cycle could be started and once confident that the fish pond was water tight I added a few marginal plants.

To provide some shade over the fish pond I built a wooden pagoda over the top of the fish pond and laid paving slabs around the outside so that access to all sides of the fish pond were easy access. Once cycled a few fish were added and the fish pond was officially up and running!

Building a Koi pond - finished pond, picture 1

Building a Koi pond - finished pond, picture 2

Question left on Mon, October 24, 2011 6:25 am by Michael

What paint brand and did you have to let mortar cure for a month first? All the ones I've seen require 28 days cure first. Also, inside is rounded?

Thank you!

Answer by staff: Hi there, I used the G4 pond paints but there are many brands to choose from, they need to form a water proof seal and be aquatic safe which means that they are not the cheapest paints but last for years. As regards curing the mortar, this is to allow the lime to leach out and let the mortar dry completely, I found that my preferred method was to allow the mortar to dry out for a few days and then coat it with PVA glue, the PVA also seals the mortar so that it cannot leach toxins into the water and makes a good base for the pond paints, I used two coats of clear G4 followed by two coats of green G4.

The corners of the fish pond are rounded below the water line to prevent any dead spots in the current.
Question left on Sun, April 8, 2012 23:59 by Karl Thompson

Concreted the pond, put fish in next day, fished died. What did we do wrong? Used easy crete. Value your advise.

Answer by staff: I have never used easycrete but what is the recommended drying time for this product as 1 day seems a bit minimal. You also need to cycle the filters in your pond by adding some bacterial culture so that the fish waste does not build up levels of ammonia or nitrites in the water. I would also imagine that there would be a high level of chlorine in the water as well as this takes a few days to drop unless the water is treated with a conditioner. We have an article on fishponds.biz that fully explains how to cycle your fish pond in preparation for the fish being added.
Question left on Sat, 3 Aug 2013 12:42:30 by Jackson Omubo

In building concrete Pond as you narrated, what did you used to reinforce the edges of the Pond?

Answer by staff: Hi

When I mixed the concrete I reinforced it with fibre glass wool. I used this mix on all of the walls as well as the edges.