Container ponds2026-10-04

How to make a container water garden or patio pond

A container water garden is any watertight pot, half barrel or stock tank that holds 12 to 36 inches of water, set in four to six hours of sun and planted so leaves cover about 60 to 75 percent of the surface. Water weighs 8.34 pounds per gallon, so a 30 gallon bowl holds about 250 pounds of water, and a straight-sided container 16 inches deep presses about 83 pounds on each square foot under it, twice the 40 pounds per square foot most decks are built for. In a container under 50 gallons the plants come first; it can hold one or two small fish through the summer, and they come indoors for the winter.

What the extension services say about the container

Kentucky GROW's Water Gardening in Containers module says "Anything that can hold water can be used, provided it is 12 to 36 inches deep and did not contain toxic materials at one time (nix the old oil drum)." Anything with holes or cracks "should be lined with 45-mil EPDM pond liner" with the edges sealed with silicone, stopping "about 2 inches from the top so that you cannot see the liner." It adds that "Whiskey barrels will hold water if they are soaked for a week in water to allow the wood to swell, or pre-formed plastic liners may be used." University of Illinois Extension warns that a hard plastic liner that does not fit "can collapse into the barrel and crack." Kentucky says aquatic plants need "at least 4 to 6 hours of sun per day," but that "all-day sun in a southwestern exposure can heat up the water to temperatures dangerous to fish."

How heavy the water is, and what a deck is built for

The USGS says "At 8.34 pounds per gallon, a million gallons comes in at 8,340,000 pounds," and its density table puts a cubic foot of water at 62.366 pounds at 60 degrees Fahrenheit.

Water weight for common containers, water only
Container	Gallons	Arithmetic	Water weight
The Pond Guy 22 inch square patio pond	15	15 times 8.34	125.1 lb
The Pond Guy 32 inch round patio pond	20	20 times 8.34	166.8 lb
The Pond Guy 27 inch square patio pond	27	27 times 8.34	225.2 lb
The Pond Guy 40 inch round patio pond	30	30 times 8.34	250.2 lb
Half barrel, 24 inches across inside, 16 inches of water	31.3	31.3 times 8.34	261.0 lb
Stock tank, 3 feet across	100	100 times 8.34	834 lb
Stock tank, 8 feet across and 2 feet deep	717	717 times 8.34	5,979.8 lb

The patio pond gallons are The Pond Guy's figures and the stock tanks are Pam Penick's, described below. The half barrel is our example. For a round container, multiply the diameter by itself, then by the depth and by 0.785, all in inches, and divide by 231, because NIST defines the US gallon as "128 fluid ounces (231 cubic inches)."

A straight-sided container presses on the floor under it at 62.366 pounds per square foot for each foot of water, whatever its width, so a 12 inch deep container puts about 62 pounds on each square foot, a 16 inch barrel about 83, and a 2 foot stock tank about 125. The American Wood Council's deck guide, DCA 6, sizes framing for "40 psf live load, 10 psf dead load," and says "Decks supporting large concentrated loads such as hot tubs are beyond the scope of this document." On balconies the AWC says "ASCE 7 requires 100 psf, while IRC requires 40 psf." Find out what the deck or balcony is rated for, set a heavy container over a beam or near a post rather than in the middle of a joist span, and ask the deck's builder or a structural engineer before a stock tank goes up there.

Plants, and how many

Kentucky says to "Aim to have 60 to 75 percent of the water surface covered with foliage to keep the water cool and to reduce the growth of algae." It leaves plants in their pots, stands shallow ones on bricks, and says lotus "should have the container all to themselves." The Pond Guy's mini pond page asks for "at least one plant that floats, one plant that sits near the edge of the pond, and one that is completely submerged."

A 24 inch barrel has 3.14 square feet of water, so 60 to 75 percent is 1.9 to 2.4 square feet. The Royal Horticultural Society says "Dwarf/small cultivars reach 30-60cm (1-2ft) wide," so one dwarf lily covers 0.8 to 3.1 square feet and can supply all of it. Add one or two marginals and a bunch of an oxygenator and the barrel is planted.

Fish: whether they fit, and how many

Kentucky says to start with goldfish or small fish, "adding one inch of fish per 10 gallons of water." The Pond Guy's patio pond page says "A good guideline is one inch of fish per gallon of water." The Pond Guy's small-pond fish page says "Since most container ponds are less than 50 gallons, they are unsuitable for housing koi and goldfish," and allows "around 200 gallons of water per one to two mature koi fish (or two to three goldfish.)" UF/IFAS says "Fish need room to swim, so the pond should be at least 500 gallons."

How many fish each rule allows
Rule	15 gallons	30 gallons	100 gallons
Kentucky GROW, 1 inch per 10 gallons	1.5 inches of fish	3 inches	10 inches
The Pond Guy patio page, 1 inch per gallon	15 inches	30 inches	100 inches
The Pond Guy fish page, 200 gallons per 2 to 3 goldfish	no goldfish	no goldfish	1 to 1.5 goldfish
UF/IFAS, 500 gallon minimum	no fish	no fish	no fish

The USGS says goldfish typically grow "to 120 to 220 mm SL," which is 4.7 to 8.7 inches before the tail, and lists the western mosquitofish at "6.5 cm," about 2.6 inches. At Kentucky's rate a 30 gallon bowl holds one adult mosquitofish, or one small goldfish that will outgrow it.

Check your state before buying mosquitofish. Oregon State University Extension writes that "It's against the law in Oregon to release non-native fish into any public waterway." The USGS adds that reviews of the literature "have not supported the view that Gambusia are particularly effective in reducing mosquito populations."

A pump or bubbler, and mosquitoes

Kentucky says fountains "are really not suitable for small container gardens," because they waste water, wind blows the spray out, and "water lilies do not flourish in moving water," though "small, floating, solar-powered fountains" might suit larger containers. UF/IFAS says a pump "in a larger water garden will circulate the water and make it less attractive to mosquitoes."

For larvae, Kentucky says to "add fish or Bacillus thuringiensis (Bt) rings to the water." The Mosquito Dunks label (EPA Reg. No. 6218-47) says "Safe For Use In Fish Habitats," lists water gardens, and gives a quarter dunk for 1 to 5 square feet of water and a half dunk for 5 to 25, releasing Bti "for a period of 30 days or more," so a 24 inch barrel takes a quarter. UF/IFAS also suggests you "just add water to the container until it flows over the top and flushes out the mosquito larvae." Our article on whether ponds attract mosquitoes covers the life cycle in more detail.

Wintering

Kentucky says "Fish in containers will not survive the winter, so make plans ahead of time to bring their containers inside or to house them in an aquarium until spring," and for plants, "sink the plants to the bottom of the container and move the container to a frost-free spot." Illinois says cold-tolerant plants can stay out "as long as the container can tolerate being frozen," and that goldfish "should be brought indoors before the water temperature drops too much." Ice expands, so empty ceramic and concrete pots or bring them in.

What container gardeners and contractors do

Pam Penick built galvanized stock tank ponds of 100 and 717 gallons in Austin and wrote them up on her blog Digging in 2009. She wrote, "I never feed my goldfish, letting them forage instead on mosquito larvae, algae, bugs, and the anacharis at the bottom," and "I've never noticed any problems with goldfish or gambusia dying off when placed in a galvanized tank."

On Garden Pond Forum, Brian B (2018) said of a half wine barrel, "I used no liner or sealer," and later ran "a Cascade 400 filter" as its fountain pump, with "three goldfish." In a 2013 thread, star1270 "kept up to 5 fish and plants" in each barrel through summer with a small pump and a "30% or so water change" at filter cleanings, and j.w suggested "an aerator running all the time."

In the TEAM Aquascape video Small Pond BIG Impact, the host builds an in-ground 7 by 9 foot pond "a little over 18 inches deep" near Chicago and says "can goldfish survive in 18 in of water in Chicago? And the answer is yes."

Where the sources differ

The Pond Guy's one inch per gallon rule is ten times Kentucky's and contradicts The Pond Guy's own fish page, and the patio page says nothing about weight, mosquitoes or winter. The PondTools stocking calculator uses 10 gallons per inch, the same rate as Kentucky. The forum posters run pumps in barrels Kentucky calls too small for a fountain, and they also change water. Aquascape's 18 inch pond keeps goldfish through a Chicago winter with earth around it, while a container on a deck has cold air on every side, which is why Kentucky tells container owners to bring fish in.

What to do

Get the gallons from the PondTools volume calculator (also standalone), which takes the inside length, width and water depth in feet and a shape, with "Circular or oval" for a barrel or bowl.
Multiply the gallons by 8.34, add the pot, soil and stone, and check the deck or balcony rating before you fill it.
Line or seal anything porous, skip anything that held oil or chemicals, and fill with dechlorinated water.
Plant for 60 to 75 percent cover with a dwarf lily or floaters, one or two marginals and an oxygenator.
If you add fish, enter the gallons, surface area, species, count and length in the PondTools stocking calculator (also standalone), feed lightly, and set up the winter tank before the first frost.
If you see larvae, float a quarter of a Bti dunk or flush the container over the rim.

When a container stops being enough, our first pond kit article walks through a 232 gallon in-ground build.