Koi pond supplies come down to eight things that matter and a long list of things that do not. The eight: a liner, a bottom drain and skimmer, a filter sized for fish rather than gallons, a pump you can afford to run around the clock, aeration, a UV clarifier, a dechlorinator and a test kit. Everything else is comfort or decoration. This guide walks the build in the order you buy it, with prices and a deep-dive guide for each category.
Short answer: Budget $3,000 to $8,000 for a DIY 1,500 to 3,000-gallon koi pond, before fish. Build in order: excavate, liner, plumbing, filter, pump, aeration, UV, water treatment, fish. Spend on filtration, depth and a bottom drain; save on waterfalls and rock work. Nothing under 1,000 gallons and 3 feet deep is a koi pond.
A koi pond is not the same build as a water garden
A water garden is a landscape feature: shallow, planted, rocked in, with a skimmer and a waterfall filter, stocked lightly with goldfish. A koi pond is life support for large, heavy-waste fish. It runs deeper, holds more water, has a bottom drain, keeps plants and loose rock out of the fish area, and puts a serious filter on the end of the plumbing.
You can keep koi in a water garden, and the people who do are the ones who call the shop in July with 4 ppm ammonia and a fish on the bottom. A 700-gallon rocked pond holding six koi has no margin: one hot week, one overfeed, one pump failure, and the water goes bad in hours. Decide which build you are doing before you dig, because retrofitting a bottom drain means draining the pond and pulling the liner.
What a koi pond actually costs
Prices below are 2026 US figures for equipment and materials, excluding fish. Professional installation runs $11,000 to $27,000 for typical residential builds, and a large contractor pond can pass $30,000. Your own digging and plumbing is where the savings live.
| Build | Volume (gal) | Realistic spend (USD) | What you get |
|---|---|---|---|
| Entry DIY | 1,000 to 1,500 | $2,000 to $4,000 | 45-mil EPDM, pump-fed skimmer, pressure filter with UV, air pump, 4 to 6 koi |
| Serious DIY | 2,000 to 4,000 | $4,500 to $9,000 | Bottom drain, gravity-fed filter, external pump, separate UV, aeration, 8 to 15 koi |
| Contractor build | 2,000 to 5,000 | $11,000 to $27,000 | The above plus excavation, rock work, electrics and a warranty |
| Running costs | Any | $25 to $90 per month | Pump and air electricity, food, treatments, UV lamps annually |
Two costs people underestimate: electricity, since a pump running 8,760 hours a year rewards paying up front for efficiency, and water, since a koi pond wants about 10% changed weekly.
How big, and how deep
The honest floor for koi is 1,000 gallons and 3 feet deep. At 250 gallons per adult fish, that is four koi. Anything smaller is a quarantine tub, a grow-out vat or a goldfish pond, and the old "100 to 1,000 gallons is a medium pond" framing that sat on this page was wrong for koi.
| Volume (gal) | Depth (ft) | Adult koi | Verdict |
|---|---|---|---|
| Under 1,000 | Any | 0 | Not a koi pond. Goldfish, plants, or quarantine only |
| 1,000 to 2,000 | 3 | 4 to 8 | Workable starter pond with strong filtration |
| 2,500 to 5,000 | 3.5 to 4 | 10 to 20 | The sweet spot for a first serious koi pond |
| 5,000 and up | 4 to 6 | 20+ | Stable water, jumbo growth, contractor territory |
Depth buys temperature stability, protects fish from herons and gives koi room to grow past 24 inches. Surface area drives gas exchange, which is why a wide shallow pond and a deep narrow one behave differently at 95°F on the same volume.
From the shop: Nobody has ever come back and told us their pond was too big. Plenty of customers buying their second batch of koi are already planning an expansion, because the pond they built in year one holds four fish and they want eight.
The build order, category by category
1. Excavation and layout
Mark the shape with a hose, call 811 before you dig, and check setback and fence rules with your city. Dig the deep zone first and keep the walls near vertical, which gains volume and gives predators nowhere to stand. Slope the floor toward the drain position at roughly 1 inch per foot. Compact the base, remove roots and rocks, then check the rim with a level and a straight board: 2 inches out shows as a wet edge on one side forever. Budget $600 to $1,500 for a mini excavator and a dump run, or a weekend with a shovel.
2. Underlayment and liner
We steer almost everyone to 45-mil EPDM: a flexible rubber sheet that folds into corners without a heat gun, tolerates 20 years of UV and freeze cycles, and repairs with a patch kit when a shovel finds it. PVC is cheaper, stiffer and shorter-lived outdoors. HDPE and preformed rigid shells are for small water features, not koi volume. Buy the liner in one piece where you can, with 2 feet of overlap at the rim, and always put geotextile underlayment beneath it; old carpet is the classic substitute and it holds water against the rubber. Size it as length plus twice the maximum depth plus 4 feet, two feet of overlap on each side, and the same for width. Firestone PondGard 45-mil EPDM runs around $1.75 per square foot. The EPDM pond liner guide covers sizing, seams and fish-safe grades; hardware and seam tape sit in liners and construction.
3. Plumbing, bottom drain and skimmer
This is the step you cannot add later without draining the pond, and the one beginners skip. A bottom drain sits at the lowest point and carries settled solids out continuously, before they rot into ammonia; a skimmer pulls floating debris off the surface. Run roughly 70 to 80% of your flow through the drain and 20 to 30% through the skimmer. Keep pipe diameter consistent, since a 4-inch drain necked down to 2-inch pipe is a 2-inch drain with extra cost. A 4-inch gravity drain moves about 3,600 GPH, a 3-inch about 1,800 GPH. Use swept 45s rather than sharp 90s, and fit knife-gate or ball valves where you will need to isolate the filter. Fittings and valves are in plumbing and valves, and the covers and skimmers guide covers skimmer choice.
4. Filtration
Filtration is where a koi pond is won. Mechanical separation comes first, to get solids out, then biological media where bacteria convert ammonia to nitrite and nitrite to nitrate. Nitrate is the end product: your biofilter makes it, and only water changes, plants or a bog remove it. Halve any filter's advertised gallon rating for koi, because those ratings assume a light goldfish load. A pressure filter with a built-in UV suits a 1,000 to 2,000-gallon starter pond; beyond that, a gravity-fed sieve or settlement chamber feeding a moving-bed or static media chamber saves hours of maintenance. Give the biofilter four to eight weeks to mature before stocking heavily. Compare types in the koi pond filter guide, and see the filters and media collection for units and replacement media.
5. Pump
Size the pump to turn the pond over once per hour, measured at your real head height. A GPH figure without head is meaningless: a pump rated 5,000 GPH at zero head may deliver 3,200 GPH once it is lifting 6 feet to a waterfall and pushing through 40 feet of pipe and a filter. Add the friction loss for your pipe, which in 2-inch pipe runs roughly 1 to 6 ft per 100 ft depending on flow, plus the vertical lift, plus the filter's stated loss. The pump guide has the table. External pumps cost more up front and use far less electricity than submersibles of the same output. Run the pump continuously; a filter that stops for six hours starts losing bacteria. The pump and aeration buyer's guide works through the maths, and the pond pumps collection lists what we stock.
6. Aeration
A separate air pump is the cheapest insurance in the hobby. Warm water holds less oxygen exactly when fish need more of it, and koi at 85°F are working hard. A linear diaphragm pump feeding two or three airstones costs a fraction of a water pump to run, and on a battery backup it carries a pond through a power cut. Place stones at mid-depth in winter rather than on the bottom, so you do not drag cold surface water down through the warm layer where the fish rest. Aeration will not stop an algae bloom, though: algae is driven by nutrients and light. Air pumps, stones and diffusers are in aeration products.
7. UV clarifier
A UV clarifier kills single-celled algae suspended in the water, which is what green water is. Run one and pea soup turns clear in five to seven days. It does nothing for string algae or blanketweed on the rocks, and it does not treat parasites on your fish. Size for the job: roughly 10 watts per 1,000 gallons for clarification, around 30 watts per 1,000 gallons if you want sterilizer-grade kill, and check the unit's maximum flow rate, since water moving too fast through the chamber gets a useless dose. Lamps lose output long before they stop glowing, so replace them yearly. See the UV sterilizers and clarifiers we carry.
8. Water treatment and testing
Two purchases here are not optional. A dechlorinator neutralises chlorine and chloramine on every top-up, and a liquid test kit tells you what your water is doing. Buy the liquid kit rather than strips: ammonia, nitrite, pH and KH. Bacteria starter is worth it while a new pond cycles. Beyond that, most bottles on a pond shop shelf are situational. Watch KH especially, because carbonate hardness is what stops pH swinging, and a pond that drops below about 80 ppm KH is one hot algae-heavy week away from a pH crash. Stock up from water treatments and test kits, and read the water treatments and medications guide before you buy anything else.
9. Fish
Fill the pond, dechlorinate, run everything for a week, then add fish slowly: two or three koi first, then more once ammonia and nitrite have held at zero for a fortnight with fish in the water. Stock to adult size, not the size in the bag, since a 6-inch tosai becomes a 24-inch fish. Every new koi should spend four weeks in quarantine first, a $450 to $1,100 setup that protects everything else you built. Start with our koi quarantine protocol, then browse Japanese koi for sale when the water is ready.
10. Food
One good staple pellet does most of the work, at 32 to 36% protein, in a size the fish take comfortably. Feed what they clear in five minutes. Switch to a wheat-germ or all-season formula below about 60°F, and stop feeding below 50°F, resuming above 55°F, because a koi's gut effectively stops in cold water. Colour foods work through carotenoids, and fed heavily they can yellow the white on a Kohaku, so most keepers alternate. Buy the bag size you will use in three months, since fats go rancid. The koi food and feeding guide has the temperature chart, and the koi food range covers staple through growth.
11. Covers, netting and winter
A heron will empty a small pond in a fortnight, and raccoons work the shallows. Netting is the cheapest defence, with an electric fence line as a strong second, and it also catches autumn leaves. It does not insulate the pond and it will not stop ice. For winter, the right default is to stop feeding, keep a hole open with an air stone at mid-depth, and let the fish sit dormant at 39 to 50°F. Partial heating to 55 to 60°F is the worst of both worlds. Never smash ice on a frozen pond, because the shock wave can kill fish. See covers and netting and the winter care guide.
Buy this first, buy this later, skip this
| Buy first | Buy later | Skip |
|---|---|---|
| Liner and underlayment | Second UV for sterilizer duty | Chemical pH "reducers" |
| Bottom drain and plumbing | Auto top-up valve | Ornamental fountains that fight your skimmer |
| Filter, oversized | Bakki shower or bog filter | Rock-and-gravel bottom in the koi area |
| Pump matched to real head | Backup pump on the shelf | Air-driven "pond vacuums" as a filter substitute |
| Air pump and stones | Battery backup for the air pump | Pond heater for overwintering koi |
| Liquid test kit and dechlorinator | Microscope for health checks | Test strips |
| Predator netting | Feeding ring and quality net | Barley straw as a pH buffer |
Your first 90 days
- Days 1 to 7: Fill, dechlorinate, run pump, filter, air and UV. Check for leaks at full level. Test pH and KH to learn your source water.
- Days 7 to 14: Add bacteria starter. Introduce two or three koi. Feed sparingly, once a day.
- Days 14 to 45: Test ammonia and nitrite every second day. Expect an ammonia spike, then a nitrite spike, then zero. Water-change 10 to 20% whenever either reads above zero.
- Days 45 to 60: Stable readings. Add the next fish, quarantined first. Set a weekly 10% water change habit.
- Days 60 to 90: Green water is normal in a young pond; UV plus a maturing filter sorts it. Clean the mechanical stage weekly and leave biological media alone.
The mistakes that cost the most money
Building too small. The most expensive error, because the fix is a second build. Torn between 1,200 and 2,500 gallons? Dig the bigger one.
Undersizing the filter. Gallon ratings assume goldfish. Halve them for koi, or buy the next model up and stop thinking about it.
Skipping the bottom drain. Retrofitting means draining the pond, moving the fish and lifting the liner: a weekend and a liner in year three instead of $200 in week one.
Buying the waterfall before the filter. A waterfall adds aeration and noise. It does not process ammonia, and a big one raises your head requirement and your power bill.
Stocking to the fish in front of you. Ten 6-inch koi look sparse in 1,500 gallons. Three years later they are ten 20-inch koi and the pond is overloaded.
Chasing pH. A stable pH of 8.4 is fine. Dosing acid in low-KH water is how ponds crash overnight.
Frequently asked questions
What causes high pH in a koi pond, and should I lower it?
Usually algae stripping CO2 during the day, fresh unsealed concrete, or hard source water. Leave a stable high pH alone. The variable to manage is KH: test it, and if carbonate hardness sits below about 80 ppm, raise it with a proper buffer or crushed oyster shell in the flow. Never dose an acid pH reducer, because in low-KH water it produces a crash that kills fish.
How many koi can I keep in 1,000 gallons?
Four adults with strong filtration and aeration. The working rule is 250 gallons per adult koi. Juveniles under 12 inches can be counted at roughly one inch of fish per 10 gallons, but stock for the fish they will become, not the fish in the bag.
Can goldfish live with koi?
Yes, and they are cheaper to lose while you learn. Two caveats: goldfish add real bioload to a volume that is usually already tight, and the two interbreed, producing sterile hybrids that look like neither parent.
Do I need both a bottom drain and a skimmer?
For a koi pond, yes. The drain removes settled solids; the skimmer takes floating debris before it sinks and rots. A drain-only pond grows a leaf mat, and a skimmer-only pond leaves waste on the floor, which is where your ammonia comes from.
How long before I can add koi to a new pond?
Run the system for a week, then add two or three fish and let the filter cycle for four to eight weeks, testing ammonia and nitrite every second day. Full stocking comes after both hold at zero with fish in the water.
What does a koi pond cost to run each month?
Roughly $25 to $90, mostly electricity for the pump and air, plus food, a yearly UV lamp and occasional treatments. An efficient external pump and a right-sized filter keep the bottom of that range in reach.
Where to start
Dig bigger than you planned, put the bottom drain in before the liner, and buy one size up on the filter. Those three decisions decide whether the next ten years are quiet or expensive; the rest can be upgraded on a Saturday. For a second opinion before you break ground, call the shop in San Jose or browse the koi food and pond supplies hub.