DIY Drain for Fish Koi Pond

After your pond is dug. ( Make sure the bottom is sloped to where you want to locate your drain.) Dig the hole and trench for your bottom drain and plumbing. Make sure you have extra room around the drain and plumbing.

Set your bottom drain in the hole. Use a level and check to see if the drain is level and about 1/2″  to 1″ lower than the ground around it.

Epic Pond 90 Degree Pond Bottom Drain - 2 inch
List Price: $97.88
Price: $97.88
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Attach the plumbing to the bottom drain. Again make sure the drain and plumbing are level. Mix and pour concrete around the bottom drain and the plumbing. There is a lot of pressure caused by the water on the bottom of your pond. By using concrete this will help stop the drain or plumbing from shifting when you fill your pond.

Install your underlayment and cut it around the bottom drain. Install your liner (make sure your liner is where you want it.) and cut the inside of the drain out. Apply silicone to the top of the bottom drain base. (make sure this is clean so you wont trap any dirt between the liner and the bottom drain base. Push the liner down into the silicone. (make sure the underlayment fabric is not between the liner and the bottom drain base.)

Koi Toilet I Aerated Bottom Drain - 3 Inch
List Price: $219.00
Price: $219.00
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Install the liner ring. The silicone should be between the liner and the base of the bottom drain, and also between the mounting ring and the liner. Install the top of the drain by cutting the center stem so that there is between 1/2″ and 1″ gap between the top of the drain and the liner. (Instead of measuring you can use the thickness of your fingers or your hand laying flat.

Bottom Drain Installation Guidelines.

Guidelines are not rules. All ponds and soil conditions are different. These guidelines should give you the basics you need, and may have to be modified to fit your pond design and soil conditions.

Do I Need A Bottom Drain?

Bottom drains are considered a necessity in koi ponds or water gardens greater than 1,000 gallons, where water quality and clarity are considered important. A bottom drain improves the water circulation and removes debris from the bottom of the pond reducing the maintenance of your pond. If you are planning on putting in under water lights they will highlight the suspended particles in the water. Bottom drains will greatly reduce these suspended particles making your water look clear even at night with under water lights.

What to Look For When Buying a Bottom Drain.

The best drains are made of PVC or ABS plastic, which is designed to be solvent welded together. The best drains also have the top attached to the sump with one single center stem. Drains that have the lids attach with a few outer parameter stems are more prone to clog with string algae and other larger debris. Aerated tops greatly improve the draw that the bottom drain has.

How Many Bottom Drains Do I Need?

Each drain will draw water from a 4′ to 6′ radius or an 8′ to 12′ diameter. This means you need one drain for every 8’x8′ to 12’x12′ area. This is based on proper bottom slope and water flow through your drains. Aerated bottom drains will draw better than standard drains.

Bottom Slope

The minimum slope is a 1″ drop for every linear foot. (Example would be if your pond is 10’x10′ and you are putting 1 drain in the middle. The middle would be 5″ lower than the outside edge.) The more recommended slope would be a 2″ drop for every linear foot. (Example would be if your pond is 10’x10′ and you put 1 drain in the middle. The middle would be 10″ lower than the outside edge.)


Each bottom drain should have its own dedicated pipe leading to the vortex or mechanical filter. There should be a valve on each pipe so the bottom drain can be purged to remove any sediment. The plumbing you use should be pressure rated and the connections solvent welded. This is true especially for the plumbing under the bottom of the pond. When you are digging the area for the bottom drain sump and the trenches for the pipes, dig them wide enough so you can encase the bottom drain sump and the pipes that are located under the pond with concrete.

The plumbing that is not under the pond should not be encased in concrete. There is a lot of pressure applied by the water to the base of the pond and by encasing the sump and pipes in concrete will help stop them from shifting. I recommend using 4″ plumbing on your bottom drain with a minimum water flow of 1,250 gph and a maximum of 2,000 gph. When cutting and fitting your pipes always fit them together dry before you solvent weld them together. Use as few bends as possible in your plumbing and stay away from 90-degree elbows if possible.

Attaching the Liner to the Bottom Drain.

After the bottom drain sump and plumbing is complete install the liner making sure you have it fitted well into the corners and as many wrinkles taken out of the liner as possible. If you are not sure fill the pond with a few inches of water so the liner is pulled tight and fitted to the bottom. Then drain the water and dry it off where the bottom drain is. Hold the liner down flat against the drain sump and cut the liner using the sump part of the drain as a guide.

Lift up the liner and make sure the bottom of the liner and the sump are clean. Apply a fish safe silicone to the top of the drain sump, and push the liner down onto the sump. Apply a fish safe silicone on top of the liner and then push the mounting ring down into the silicone making sure the screw holes in the sump line up with the screw holes in the mounting ring. Tighten the screws down evenly on the mounting ring. Call it a day and let the silicone set.

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