Pond fish: types, building a pond, filters and feeding

Pond fish types

Not all fish suit ponds. The ones that do are hardy, cold-tolerant, and can handle seasonal temperature swings. Here are the main options.

Goldfish (common, comet, shubunkin)

Goldfish are the most popular pond fish for good reason. They’re hardy, tolerant of a wide pH range (around 6–8), and can live 20–30 years when given space, clean water, and good care. In a pond they can reach 10–14 inches. Comets have long, flowing tails; shubunkins have calico colouring (red, black, blue, white) and are often favoured for their looks. Common goldfish are the classic single-tail variety. All do best in groups of at least three. They need a pond at least 1–2 feet deep — deeper in cold climates so they have somewhere to overwinter below the ice.

Koi

Koi are ornamental carp — larger, more demanding, and longer-lived than goldfish (50–70 years in ideal conditions). They can grow over 3 feet and need a pond of at least 1,000 gallons (3,800+ litres). Depth matters: aim for 3 feet or more so they have room to overwinter and avoid predators. Koi are less tolerant of poor water quality than goldfish; they need stable parameters and good filtration. They’re a serious commitment — beautiful, but not suited to small or shallow ponds.

Orfe (golden orfe)

Orfe are active, schooling fish that grow to around 60–75 cm. They prefer well-oxygenated water and plenty of swimming space. Keep them in groups of at least four or five. They suit larger ponds (4,500+ litres) and are often mixed with goldfish. They add movement and colour to a pond but need more room than goldfish.

Other options

Sarasa comets have red and white patterning and similar care to goldfish. Fancy goldfish (orandas, ranchus, ryukins) are better suited to aquariums — their rounded bodies and delicate fins make them vulnerable to predators and poor in strong currents. For ponds, stick to single-tail, streamlined varieties.

How to build a pond

Building a pond is a weekend project for a small one, or a larger undertaking for a fish pond. Here’s the framework.

Depth

Depth is critical. For goldfish and similar species, aim for at least 2 feet (60 cm) — deeper in cold climates so part of the pond stays unfrozen in winter. For koi, 3 feet (90 cm) or more is recommended. In very cold regions, 4 feet helps ensure fish have a refuge below the ice. Shallow ponds heat up quickly in summer and cool too fast in winter; fish can suffer from temperature swings and lack of oxygen.

Size and shape

There’s no single “right” size — but a fish pond needs enough volume to dilute waste and maintain stable water chemistry. A rough stocking guide: one inch of fish length per square foot of surface area for new ponds; mature, well-established ponds with plants and good filtration can support more. Start conservative. Add shelves at 1–2 feet below the surface for marginal plants; the deep area is for fish and overwintering.

Liner options

Preformed plastic liners are quick to install but limit shape and size. Flexible PVC or EPDM rubber liners give you freedom to dig any shape. EPDM is durable and long-lasting. Calculate liner size: length + (depth × 2) + overlap; same for width. Add a few inches for overlap at the edges.

Basic build steps

1. Mark out the shape and dig — shelves for plants, deeper centre for fish.
2. Remove sharp stones and roots; add a sand underlay if desired.
3. Lay the liner, fold edges neatly, and secure with rocks or edging.
4. Fill with water, add dechlorinator, and let it sit for a few days before adding fish.
5. Add plants if you want them — they help with oxygen and shade.

Filter or not?

Whether you need a filter depends on what you’re keeping.

Ponds with few or no fish

Wildlife ponds, small water features, or ponds with very light stocking (e.g. a few small goldfish in a large volume) can often get by without a filter. Plants, natural bacteria, and water changes help. The water may not be crystal clear but it can be healthy.

Ponds with fish

Once you add fish in meaningful numbers, waste builds up. Ammonia and nitrite are toxic; nitrate accumulates. A filter provides mechanical removal of debris and biological filtration — bacteria that convert ammonia to nitrite and nitrate. For goldfish ponds, a filter is strongly recommended. For koi, it’s essential.

Filter types

• Internal filters — Sit inside the pond. Suit smaller ponds with light stocking.
• External (gravity or pressurized) — Sit outside the pond; water is pumped in and returned. Better for larger ponds and heavier stocking.
• Mechanical + biological — Most filters combine both: foam or mesh removes solids; media hosts bacteria for the nitrogen cycle. Biological filtration takes several weeks to establish — similar to cycling an aquarium.

Size the filter for your pond volume and fish load. Manufacturers often rate filters by volume; aim for one rated for your pond size or slightly larger when stocking fish.

Aeration

Pumps, fountains, or air stones add oxygen and help water circulate. In summer, warm water holds less oxygen; in winter, ice can seal the surface. Aeration helps in both seasons. A simple pump or fountain can double as aeration.

Feeding pond fish

Feeding should follow water temperature, not the calendar.

When to feed

In warm months (water consistently above 50–55 °F / 10–13 °C), fish are active and metabolism is high. Feed once or twice daily, only what they can eat in a few minutes. Overfeeding leads to waste, ammonia, and algae. In cooler weather (below 50 °F), reduce feeding. When water drops below 40–45 °F (4–7 °C), stop feeding. Fish enter torpor; their digestion slows and uneaten food can cause problems. They can safely go months without food in winter.

What to feed

Use a pond pellet or stick formulated for goldfish or koi. Staple foods work year-round when you feed. In summer, growth or colour-enhancing foods can be used occasionally. Avoid feeding bread or human food — it can cause digestive issues and pollute the water.

Spring

When water consistently stays above 40 °F for a week or more, start feeding again gradually — every other day at first, then daily as temperatures rise. Fish may be sluggish after winter; don’t overfeed.

Quick takeaways

• Goldfish (comet, shubunkin, common) are the most practical pond fish; koi need larger, deeper ponds.
• Pond depth: at least 2 feet for goldfish, 3+ feet for koi; deeper in cold climates.
• Filters are recommended for fish ponds; essential for koi. Size for volume and stocking.
• Feed by temperature: stop below 40–45 °F; feed once or twice daily when warm. Only what they eat in minutes.