Bottom drains are advantageous in koi ponds and water gardens where water quality and clarity are important.

In any water garden or koi pond, water circulation is an important consideration. The pump is the heart of the pond circulation and filtration system. The water can either be drawn directly to the pump when it is sitting on the bottom of the pond, or the water can be drawn through pipes and filter boxes from the surface or from the bottom of the pond.

In a small pond, a pump on the bottom can be reached easily. This allows the pond owner to clean the pump when it clogs with debris. In deeper ponds, larger pumps move a lot of water and are therefore more susceptible to clogging. Any pump or filter in the pond is harder to hide. The pipe from the pump to a waterfall or filter can be hard to hide.

It is easier to maintain a pump located outside the pond, while still getting flow from the bottom from a bottom drain. Some bottom drains are connected through a hole in the bottom of the pond and allow for the full drainage of the pond. Other bottom drains use water pressure to push and pull the water to an opening in the liner close to the surface.

How a Bottom Drain Works
Some people picture bottom drains like the drain in their bathtub, but bottom drains in ponds are connected to filters. Their primary purpose is not to drain the pond completely of water, but to ensure optimal circulation of all the water in the pond through the pond filters.

Bottom drains work by simply physically connecting the drain to an external filter box and then raising the pond water level above the filter using a pump. Since water always seeks its own level, water from the pond will flow into the filters or skimmer box as long as the pump is running. A bottom drain connected to a skimmer box, an external pond pump, or a settling chamber will always drain from the higher pond level to the lower water level in the filters. Regardless of using a gravity-fed drain to settling tanks or a powered skimmer drain, bottom drains work because the pump lowers the water level in the filter and the return water from the pump raises the water in the pond above the water level in the filter.

The circulation of the pond water is important because it runs the water through the filter system. The filters remove large and small particles and invisible chemicals that could build up to toxic levels. Bottom drains help by moving water and debris from a part of the pond not easily accessible. Some promoters of bottom drains have insisted they are essential for ammonia reduction, claiming ammonia is heavier that water and will accumulate in the bottom of the tank. This theory is really a myth, but bottom drains are still useful.

Kio ponds and water gardens are at the opposite ends of a spectrum of pond designs. Koi ponds are typically larger, deeper, and have no rocks on the liner. They are often shaped like the diving end of a swimming pool. Water gardens are usually less that 3 feet deep, have shallow shelves, and are often covered in gravel and boulders.

Bottom Drains in Koi Ponds
Koi ponds are designed for koi keeping and as such, high water quality is the number-one goal. Large external filters are the norm and bottom drains in bare bottoms are considered a necessity to deal with large fish loads and optimum water circulation. Koi ponds usually have smooth, bare liner or concrete bottoms, so debris can be easily removed as it slides down along the bottom slope toward the drain. Drain sizes vary from 2" to 4" in diameter depending on the pumps used and pond volume.

The AKCA Guide to Pond Construction recommends a minimum bottom slope of 1" drop per one linear foot, but a 2" drop/foot is preferred. This guide notes that a bottom drain with the proper slope will draw water from a 4' to 6' radius. Multiple drains should therefore be used based on this area guide so all parts of the pond are drained adequately.

Bottom Drains in Water Gardens
If bottom drains are so important, why would anyone ever want to build a pond without one? Answer: Not everyone wants a deep, bare-bottom koi pond, and many water gardens can support casual koi keeping effectively without a bottom drain. It never hurts to use a bottom drain in any pond, but having seen hundreds of successful water gardens without bottom drains, I can attest to the fact that fish can survive without them. However, "survive" and "thrive" are perhaps not the same thing when it comes to pond without bottom drains. This simple fact will always remain: Water quality is king as far as the fish are concerned. While plants can thrive without a bottom drain, fish are more apt to thrive with one.

In water gardens, plants help greatly with filtration, powered skimmers can collect perhaps ninety percent of the debris from the surface of the pond, and a gravel sub-base in the garden pond on top of the liner supports a thriving culture of bacteria to break down fish waste. These design elements in a water garden provide additional filtration, minimizing the necessity of a bottom drain. But if your water garden is 3' deep or more, you'd like to add koi, or your fish keep growing in size or number, then even water gardeners should consider a bottom drain. I would not hesitate to install a bottom drain on even a 18" to 30" deep pond because of the multiple benefits to improved water circulation.

For example, bottom drains in water gardens improve water clarity, particularly nighttime clarity. I use powered skimmers in every pond I design or build because they are essential for maintenance, appearance, pump protection, and water quality. For years I shunned the use of bottom drains on water gardens because of the added complexity and fear of leaks, but always noticed that my gin-clear, completely spotless, seemingly drinkable water in the daylight, was filled with little floating things lit up brightly at night from the underwater lights. A normal skimmer doesn't pull water out of the middle or bottom of the pond to filter out these suspended particles. If you value crystal-clear water at night, then even water gardeners should consider a bottom drain. You will immediately notice the difference in water clarity and greatly assist your filters in capturing all the pond water for optimal filtering and water quality.

I added a 2" diameter bottom drain to the powered skimmer in my own 30" deep water garden and was, quite frankly, stunned at the improvement in water clarity. Side benefit: I also keep koi in my water gardens and the bottom drain gives better water circulation, which equals bigger koi, healthier koi, and longer-lived koi. I like all of that. And the price- the cost of the plumbing and the small amount of labor- is a small one to pay for better water clarity and better pond circulation.

Bottom Drain Choices
Several factors will weigh in on what type of bottom drain you select and how you install it. If you're doing a retrofit to an existing pond and prefer to avoid re-digging the pond, an above-the-liner drain will be your choice. I personally prefer that type for even new pond construction to simplify future maintenance of the bottom drain plumbing and simplify leak detection. Drains that work without cutting a hole in the bottom of the liner at the drain connection are for anyone who worries about leaks sometime in the future. Potential leaks can be avoided by simply following tried and true installation techniques.

Another choice if using an above-the-liner type drain is whether to use a vacuum drain or a drain raised a couple of inches off the bottom. A vacuum drain is preferred for bare-liner ponds, as it can pull debris right off the top of the liner. A slightly raised bottom drain is preferred when adding a gravel/rock substrate liner covering. Do not cover a drain with gravel as flow would be restricted, and inspection for debris would be more difficult.

All drains should be equipped with either perforated holes or domed covers, which keep fish out but let waste freely pass without clogging. Domed drain covers should be designed so their support legs do not interfere with debris flow.

Piping Advise
The koi pond filter typically has a bottom drain feeding into a mechanical settling tank. This is a preferred drain plumbing method so the debris is removed from the water before the pump returns the clean water back to the pond. Feeding a bottom drain into a powered skimmer box accomplishes the same objective by filtering the dirty water through mats before it reaches the pump. Flexible PVC offers some advantages over rigid PVC such as freedom from ninety-degree bends, freeze crack resistance, and elimination of some glue joints. We use flex PVC to make quick work of inside-the-pond bottom drains on water gardens. If you're looking for a 4 inch diameter drain, however, flex PVC may be a little hard to find. The AKCA guide recommends ABS in 3" and 4" diameters, and claims toxicity concerns with this material are not valid.

On water garden pond designs the plumbing for the bottom drain penetrates the liner only about a foot below the water's surface and adjacent to the skimmer box. This simplified plumbing technique avoids the possibility of leaks around bottom drain fittings that require cutting the liner in the bottom of the pond. If any leaks occur with this style of plumbing, the leak can be determined since it will be located outside the perimeter of the pond, and should be readily repairable without having to drain the entire pond.

Another benefit to this style of bottom drain is that a single pump can be used to create a dual-flow pond filtration system, pulling high oxygen top surface waters and debris and mixing them with lower oxygen bottom pond water. In a dual-flow circulation system like this, both top debris that blows into the pond and sunken debris are removed simultaneously, without risking pump clogging.

If your skimmer box is equipped with a bellows device to close the door opening, all the water entering the skimmer box will be forced in from the bottom drain. This increases flow dramatically, cleaning the drain and allowing the flow to be checked for a normal rate, to assure no drain blockage has occurred. By using vertical mats in the skimmer box, the bottom drain enters the box low in the front filter chamber. This increases the drain flow whenever the bellows is closed and prefilters the dirty water before it can reach the submerged or external pump that powers the skimmer. Add a gate valve to the drain line in order to close the bottom drain when servicing the skimmer box or when colder temperatures force you to avoid mixing the warmer bottom water with colder top water in the pond.

Conclusions
Bottom drains are a must for serious koikeepers and advantageous for many water gardens where water quality and clarity are important. They can be added during initial construction but retrofits are very easily accomplished with above-the-liner plumbing and designs available from many different vendors. Worries about leaks from penetrating the liner can be minimized by carrying the drain line to the outer wall of the pond and using bulkhead fittings or other tried and true liner or concrete penetration techniques available from reputable supply dealers and pond installers everywhere.

From my experiences and hundreds of clients who have tried bottom drains, I can definitely say, "Try it, you'll like it!"