Whether you have a new house with a backyard or are just trying to renovate an old house with gardens and an empty lawn, a fishpond is always a good idea to improve the ecosystem. Building an underground fishpond is easier and cheaper than you think. The result will be surprisingly cool, fresh water in which happy fish swim back and forth.

How to Build An Inground Fish Pond (Ultimate Guide)

Planning and design of an underground pond

These preparatory steps are essential to avoid setbacks or disappointments in the implementation process.

Pond place

The first step is to determine where your pond will be in your garden. The optimal location would be a sufficiently large space with full/partial sunshine in the morning and some shade in the afternoon. This is to prevent algae from overcrowding your pond. In addition, no mature trees or hard tree trunks, roots, etc. should be present to prevent perforation of the pond liner.

There must be a GFCI outlet for a filter pump and a waterfall or pond fountain next to the pond.

Also make sure you call the number of your local utility company to ensure that no pipes or cables are laid underground in the area of the future pond.

Pond shape, size and depth

Now plan and calculate the shape and size of the pond. Do you have a pond at your disposal or would you like to modify it yourself? In any case, it is preferable to stretch the shape of the pond and take a virtualised measurement before digging. You can use anything you want, from cheap old ropes, ropes, etc. to temporary marking paint to mark the desired shape of the pond on the ground.

Do not forget to measure and record the expected measurement of your pond. You need these numbers to select components such as pumps, coatings, etc. If you plan to build an underground pond with fish and plants, plan at least 2.5 metres by 2.5 metres. To be honest, in reality a pond can never be too big, because fish need space to grow and be healthy.

An important observation concerns the depth of the pond. In general, the pond should be at least two metres deep for small fish such as goldfish, minnows, fur coats, etc. The pond must be at least two metres deep. For koi and other large fish, add 1 to 2 feet to the deposit. If you are hibernating, dig an extra 1 meter hole in the middle to help the fish survive the frost.

What do you need for an underground fishpond or pond parts

So once you’ve designed the pond, it’s time to buy everything you need. Don’t forget that all the features you add to the pond must now be planned and prepared in advance for the entire system to work perfectly.

Pond liners

The pond liner is the basis of every beautiful and active fish pond. It is very important that your water is free of dirt and it is clear that you should spend most of your budget on purchasing a high quality pond liner. Over time you can repair most things that went wrong, but if the pond liner is cracked or leaking, it will take a lot of time, luck and common sense to empty the pond, keep your fish and find the damaged area.

It is advisable to use a 45 mm EPDM pool liner as this material is more resistant to UV radiation, perforation and other interferences. Lightweight PVC pond liners are also available if you want a small pond in your garden.

Whichever liner you use, make sure it is large enough to cover the pond, depth and an extra foot from the edge of the pond. Also make sure that the material of the pond liner is safe for fish and live animals.

Payment for basin

The pond liner is designed to protect the pond liner from cracks or bumps caused by tree roots, sharp stones and other objects. There are many ways to do this. You can pay at the suppliers of ponds; they are generally strong, flexible and very easy to manage.

You can use a piece of carpet residue as an underpayment under the pond liner for a less expensive option.

Pond skimmer

Each pond must be maintained at a certain time, daily, weekly, monthly or seasonal. A common problem is floating debris, leaves or objects that make your pond less clean. There’s a pond skimmer to help solve this problem.

When constructing an underground pond, in principle a pond skimmer is installed. Inside is a submersible pump that helps to suck the surface water into the skimmer and collect floating objects in the inner basket. The skimmer should now be cleaned from time to time instead of cleaning the entire pond daily in a large pond.

Flexible PVC hoses

You need some kind of hose to bring water from the pumps to your taps. Choose rolled PVC tubing for ease of milling and durability.

Submersible pump

The submersible water pump can be placed in the pond skimmer to attract floating objects, or at the deepest point of the pond to suck up water containing debris, silt and sinking waste.

Depending on the size of your pond, it may be necessary to regularly clean the filter of the pump. Pump filters are often more difficult to clean because the pump has to be opened and the filter carefully removed. It is therefore advisable to install a pre-filter to collect coarse dirt, sludge and fish waste. Most pre-filters can be easily cleaned by flushing them out of the hand shower with the hose.

For the skimmer option, a submersible pump must be installed during the construction phase; however, it can be purchased for waste and solid waste and added later.

To buy the right pump for your pond, look for pumps that match the size of your pond. Even if you have to climb several steps, it depends on how many things are connected between the pump and the water connections, bypasses, etc.

Pond filter

Your pond comes with different types of filters, e.g. a pre-filter, pressure filter, UV filter, etc..

The idea is to keep the water as clean and clear as possible so that your fish are healthy and have fun. Some filters perform better and worse when serviced, because every additional filter you put into service requires some maintenance.

The pre-filter acts as a connection for solids and waste and prevents them from blocking your submersible pump. A pressure filter is required for small particles. The pressure filter usually has an integrated backwash system, which helps maintain hygiene thanks to the different pumps on the handle.


The ioniser in the pond releases copper ions into the water via a probe to kill the algae strands. This will make your water cleaner and more pleasant.

Normally the ioniser anode needs to be replaced annually to maintain operational stability. You should also test the copper concentration of the water to ensure that the fish are not damaged. For more information about the operation of the ionizer according to the water level in your pond, see the Pond Guide.


Aeration helps to produce oxygen for your pond to keep the fish alive. Normally the deepest part of the pond contains less oxygen and tends to lose more oxygen at night or during the cold winter months. Therefore, each livestock tank must be equipped with one of these tanks.

Grocery pond or water heater

If you have a pond with fish in a cold environment in winter, a pond heater or heater will help prevent the pond from freezing completely and the fish from dying in winter. As a rule, fish can hibernate and survive in the deepest part of the pond if the entire surface of the pond is not frozen.

Reindeer generally have a power output of about 300 watts, which means they can be turned on throughout the winter season. For better temperature control look for thermostats with built-in thermostats that are safe for your pond liner.

When you have collected everything, check the weather and choose a good day or days when you can do all the handiwork.

Step-by-step instructions for the construction of an underground fishpond

1. Pond preparation and Sketch

As mentioned above, take the first step in planning and buy everything you need. Don’t forget to call the number of the excavation and utility company to make sure there are no pipes, tubes or other underground works in the area of the pond that you have marked. Draw a drawing of where permanent components such as the foam separator, waterfall, sewerage, etc. should be installed.

Also use marking paint, long ropes or strings to indicate the shape of the pond at the bottom of the plot.

Whatever you need: Marking paint, ropes or anything else that marks the excavation site.

2. Dredging ponds, pits, racks, etc., as follows

Dig your demarcated area on a nice sunny day or on sunny days. If it’s full of grass, maybe you should lift it. If your floor is stuck to a shovel, spray water with a hose and it will fall off immediately.

How to Build An Inground Fish Pond (Ultimate Guide)

Start digging the entire marked area to a depth of 1 foot. Then go a meter around the perimeter and dig deeper. So when you’re done, the pond looks like a staircase, with benches and shelves for the plants and easy access.

Don’t forget to hold the excavated soil and send it to the landfill or sidewalk.

Whatever you need: Shovel, water hose, axe, gloves, pottery container…

3. Installation of fixed components: Skimmer, drain, canals, waterfall…

This is an extra step if your pond is large and you want to install fixed elements such as skimmers in front of the pond, a gutter under the pond, gutter filling or waterfall pipes… Depending on the desired function, place your parts in the right place, align them properly and follow the production instructions.

4. Preparation of the Foundation: Sand filler and pond bottom

The pond foundation does not look smooth and safe when the pond liner runs over it, so it must be vacuumed carefully and everything must be vacuumed. Make sure the pond bank is level on all sides; if not, use the excavated soil to create the bank.

If you come across a large stone or a sharp object, make sure you remove it. If there are a lot of pebbles and small pebbles under the pond, you can buy construction sand and level it at the bottom of the pond to about 3 pebbles.

However, place a piece of pond liner under the payment or a pond mat to protect your pond liner. Do this to prevent leaks and unnecessary stress on the ship.

Whatever you need: Pond or carpet, construction sand.

4. Install the pond liner

The pond liner should cover every inch of your pond and an extra meter around the edge of the pond. You may need a helping hand to position the pond liner, cover it, shift the angle, etc. It is also useful to fill the pond to a third to half of its capacity to ensure that the foil is stretched to the bottom.

Also make sure that the pond liner has a lid and skimmers on top of each other, a drain tray, the bottom surface of the waterfall, if you have installed one.

Once the panelling is in place, cut the extra corner with a knife or scissors.

Whatever you need: Pond liner, water hose, utility knife.

5. Rocks, pebbles and gravel in

Now it’s a pleasure to decorate and place all the stones, pebbles and boulders. Start with the large blocks to go around the perimeter of the pond, fix the extra space in the pond liner, fill it with smaller stones and finally cover it with pebbles.

Do the same to decorate the waterfall. It may be necessary to seal them with foam to prevent backflow or leakage.

Anything you need: Stones, pebbles, pond moss.

6. Waste-water treatment plant: Filter, aerator, UV-filter, ionizer…

In order for your fish to survive and grow healthily, you must ensure that the water is clean, contains sufficient oxygen and is at the right temperature.

You need filters to keep the water clean and clean. Sludge or a solid pre-filter at the bottom of the pond will help to keep the pump and pond free of excess waste. If you have a tree with lots of leaves and other particles near your pond, you may also need a pressure filter to remove all mud and small objects.

The addition of an aerator allows you to supply your pond with sufficient oxygen at night and during the cold winter months with minimal energy costs.

A pond in a cold environment may need a deer or a stove to prevent the fish from freezing completely and thus hibernating.

Connecting an ionizer or UV filter will help ponds with algae and green water problems enormously, but you will have to set and adjust the amount of copper ions used.

7. Filling the pond

Now that your pond is completely built and finished, it’s time to fill it with water. You have two possibilities to fill up with city water or natural water from a rainfall or spring. In the first case, you should dechlorinate the water by adding a commercially available dechlorination agent or by filling and closing the pond water for one or two days.

How to Build An Inground Fish Pond (Ultimate Guide)

If you have a natural well or if it rains a lot in your area, you can fill up this water immediately and use it without chlorine. A temporary connection of the hose to a gutter in a large pond will significantly reduce water consumption.

In any case, you should let your pond rest for a while after filling it with water, so that it can regain its temperature.

8. Addressees: Fish, plants and other aids

If the water temperature is correct, it is time to add fish, plants and other equipment. Search for species that can be found in your area. Don’t overfeed them and take a few plants with you to give your pond a show, shade and beauty.

How to Build An Inground Fish Pond (Ultimate Guide)

Most water plants can grow very well on a flatter bench that you have made in advance; make sure that the water temperature corresponds to their former habitat.

Keep your fish and plants healthy and happy in a clean pond and enjoy your creation for years to come.

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