How to Build a Goldfish Pond: Size, Depth and Filtration

A goldfish pond is the easiest fish pond you can build. Goldfish stay far smaller than koi, tolerate a wide temperature range and need a fraction of the filtration, so a small liner pond in a corner of the yard can keep them healthy for a decade or more. The essentials: aim for at least 300 to 500 gallons, make one zone at least 24 inches deep (30 inches or more where winters freeze hard), move the full volume through a filter about once an hour, cover half the surface with plants and stock lightly. This guide walks through each decision for a small, goldfish-specific outdoor fish pond, from picking the spot to adding the first fish.

Part of our Ponds & Water Gardens guide →

In this guide

1. Goldfish Pond vs Koi Pond: Why the Plan Is Different
2. Step 1: Pick the Right Spot
3. Step 2: Decide Size and Depth
4. Step 3: Size the Pump and Filter
5. Step 4: Dig and Shape the Hole
6. Step 5: Lay Underlayment and Liner
7. Step 6: Edging, Plumbing and Hiding Places
8. Step 7: Plant Before You Stock
9. Step 8: Cycle the Water and Add Fish
10. Feeding and Seasonal Care
11. Typical Materials List
12. Frequently Asked Questions

Goldfish Pond vs Koi Pond: Why the Plan Is Different

Comets, shubunkins and sarasa goldfish typically reach 6 to 10 inches outdoors. Koi routinely exceed 2 feet and produce much more waste. That difference changes everything: a goldfish pond can be shallower, smaller and run on a modest pump and pressurized or box filter, while a koi pond needs thousands of gallons, bottom drains and heavy-duty filtration. If you think you might want koi later, build to koi standards from the start; our koi pond building guide covers that larger project. If you want a general-purpose water garden with or without fish, see how to build a backyard pond. This guide stays focused on a small pond built around goldfish; for everything else, from plants to seasonal upkeep, start with our ponds and water gardens guide.

Stick to single-tail varieties outdoors. Fancy goldfish such as orandas, ryukins and telescopes are slower swimmers with delicate fins and do poorly with predators, cold water and competition from comets.

Step 1: Pick the Right Spot

• Sun: Four to six hours of direct sun is ideal. Full all-day sun drives algae and summer heat; deep shade limits water lilies and invites leaf litter.
• Away from trees: Leaves decay into sludge and roots can puncture liners. Keep the pond at least the canopy width away from large deciduous trees.
• Not in a low spot: Runoff carries fertilizer, mulch and lawn chemicals into the pond. Build on level or slightly raised ground.
• Near power and water: You need a GFCI-protected outdoor outlet within reach of the pump cord, and a hose to top off evaporation.
• Visible from the house: You will enjoy the fish more and spot problems faster.

Call 811 at least a few days before digging so buried utility lines are marked. Check whether your town treats ponds deeper than a certain depth, often 18 or 24 inches, as requiring a fence or permit.

Step 2: Decide Size and Depth

Bigger water is more stable water. Temperature, oxygen and ammonia all swing faster in a small pond, which is why 100-gallon tubs struggle to keep fish healthy through a hot summer. For a dedicated goldfish pond, plan on at least 300 gallons, with 500 to 1,000 gallons as the sweet spot. That is roughly 6 by 8 feet at 2 feet deep on the small end.

USDA zone	Minimum deep-zone depth	Why
3 to 4	30 to 36 inches	Ice can be thick; fish need unfrozen water below
5 to 7	24 to 30 inches	Seasonal ice; deep zone stays near 39 degrees F
8 to 10	18 to 24 inches	Depth mainly buffers summer heat and deters herons

Only part of the pond needs to be that deep. A good profile has a plant shelf 8 to 12 inches below the waterline around much of the edge and a deep zone covering about a third of the floor. Shelves hold marginal plants; the deep zone shelters fish from heat, cold and predators. For more on depth trade-offs, see how deep a pond should be.

Volume and liner math

Gallons = length (ft) x width (ft) x average depth (ft) x 7.48. An 8 by 6 foot pond averaging 1.5 feet deep holds about 540 gallons. Liner size = length + (2 x maximum depth) + 2 feet for overlap, and the same for width. For that 8 by 6 pond with a 2-foot deep zone, buy a liner at least 14 by 12 feet. Use 45-mil EPDM or a fish-safe reinforced polyethylene liner, and always put a geotextile underlayment beneath it.

Step 3: Size the Pump and Filter

For goldfish, the target is to pass the full pond volume through the filter about once per hour. A 500-gallon pond needs a pump delivering around 500 gallons per hour at the height it has to lift water, not the zero-head figure printed large on the box. If the water climbs 3 feet to a waterfall or filter box, read the flow at 3 feet of head on the pump chart and buy accordingly. Our guide to choosing a pond pump explains head height and tubing losses in detail.

Pond volume	Pump flow at working head	Filter rated for
300 gallons	300 to 450 GPH	At least 600 gallons with fish
500 gallons	500 to 750 GPH	At least 1,000 gallons with fish
1,000 gallons	1,000 to 1,500 GPH	At least 2,000 gallons with fish

Filter manufacturers usually list two ratings: a larger number for ponds without fish and a smaller one for stocked ponds. Use the stocked rating and round up. For a small goldfish pond, three filter styles work well:

• Pressurized filter with UV: Buries beside the pond, pushes water to a waterfall or return, and often includes a UV clarifier for green water. Easy to hide.
• Waterfall filter box: Sits at the top of a small falls; water rises through mats and bio-media and spills back. Great if you want moving water.
• Submersible filter: A box on the pond floor with the pump attached. Cheapest and simplest, but you must lift it out to clean it.

A skimmer is optional in a small goldfish pond but helps a lot if trees are nearby. Whatever you choose, include an aerator or a splashing return in summer; warm water holds less oxygen. You can compare current options with this pond pump and filter search on Amazon, or read our best pond pumps roundup.

Step 4: Dig and Shape the Hole

1. Outline the shape with a hose or rope. Simple kidney or oval shapes are easier to line than tight curves.
2. Dig the shelf first. Remove soil across the whole outline to 8 to 12 inches deep, leaving a shelf 10 to 12 inches wide around the edge.
3. Dig the deep zone inside the shelf to your target depth. Slope the walls slightly outward, about 20 degrees, so soil does not collapse and ice can push up rather than out.
4. Level the rim. Lay a straight board across the pond with a level on top and check every few feet. A rim that is 1 inch low on one side will show liner and lose water there.
5. Remove rocks and roots, then add 1 to 2 inches of sand on the floor and shelves.
6. Dig a trench 3 to 4 inches deep and 6 inches out from the rim if you plan to cap the edge with stone; the liner will tuck into it.

Step 5: Lay Underlayment and Liner

Spread geotextile underlayment over the entire hole, overlapping seams by 6 inches. Unfold the liner on a warm day when it is pliable, center it, and let it settle into the hole. Pleat the excess into a few large folds rather than many small wrinkles. Start filling with a hose while you smooth the liner, pulling folds tight as the water weight pulls it into the corners. Leave the edges long until the pond is full, then trim to about 12 inches past the rim.

Step 6: Edging, Plumbing and Hiding Places

Cap the rim with flat flagstone or rounded river rock, overlapping the water slightly so it shades the liner from UV and hides it from view. Set the pump on a brick or flat stone in the deep zone so it does not suck up debris from the floor. Run the tubing to the filter and return, and route the cord through a buried conduit to a GFCI outlet.

Add at least one fish cave. A short length of 6- or 8-inch PVC pipe or a stack of flat stones with a gap underneath gives goldfish a refuge from herons and raccoons. Herons wade in shallow water, so steep sides with no shallow beach near the deep zone also help.

Step 7: Plant Before You Stock

Plants are part of the filtration in a goldfish pond. They take up nitrogen and phosphorus that would otherwise feed algae, shade the water, and give fish cover. Aim to cover 50 to 60 percent of the surface by midsummer.

• Water lilies in the deep zone provide shade and surface cover. A hardy lily in a 2- to 3-gallon basket suits a small pond.
• Marginals on the shelves, such as pickerel rush, dwarf cattail, iris and creeping Jenny, soak up nutrients.
• Submerged oxygenators like hornwort compete with algae and provide spawning cover. Goldfish will nibble them, which is fine.
• Floaters such as water lettuce provide instant shade in warm zones, but check local rules; some floating plants are restricted or invasive in southern states.

Step 8: Cycle the Water and Add Fish

Treat tap water with a dechlorinator that also neutralizes chloramine; many cities use chloramine, which does not gas off on its own. Start the pump and filter, add beneficial bacteria if you like, and let the system run for two to four weeks with plants in place. Test for ammonia and nitrite; both should read zero before fish go in.

Add only a few fish at first, floating the bag for 15 to 20 minutes to equalize temperature, then letting pond water mix in gradually. Add more over several weeks so bacteria can keep up. A safe long-term stocking level for a lightly filtered pond is roughly one adult goldfish per 50 gallons. Goldfish breed readily, so expect to rehome some fish every few years.

Feeding and Seasonal Care

Water temperature	Feeding	Main tasks
Above 70 degrees F	Floating pellets once or twice daily, only what they eat in 3 to 5 minutes	Top off evaporation, add aeration, rinse filter mats in pond water
50 to 70 degrees F	Once daily or every other day; wheat-germ food in cool water	Divide plants in spring, net leaves in fall
Below 50 degrees F	Stop feeding	Remove pump from deep zone or raise it, keep a hole in the ice with a de-icer or bubbler

Overfeeding causes most goldfish pond problems: uneaten food rots, ammonia rises, and algae explodes. Fish in a planted pond also graze algae and insects, so they need less than you think. In fall, net the pond against leaves and cut back marginals. In winter, never break ice by hitting it; the shock can harm fish. Use a floating de-icer or an air pump to keep a vent open for gas exchange.

Typical Materials List

• 45-mil EPDM or fish-safe liner sized with the formula above
• Geotextile underlayment the same size as the liner
• Pump rated for one turnover per hour at working head
• Filter rated for at least twice the pond volume with fish, ideally with UV
• Flexible pond tubing and clamps sized to the pump outlet
• Edging stone, sand and a few flat stones for caves
• Dechlorinator, test kit for ammonia, nitrite and pH
• Lily, marginals and oxygenating plants with aquatic baskets and soil
• Pond net for fall and a de-icer or air pump for winter in cold zones