Complete Guide to Earth Pond and Earth Floor Goldfish Cultivation | Natural Filtration, Green Water Management, and Breeder-Style Rearing Methods

Earth Pond and Earth Floor Goldfish Cultivation

Earth pond (doike) and earth floor (doma) goldfish cultivation refers to goldfish rearing in large-scale spaces using natural soil—clay-based soil or Arakida soil—as substrate rather than concrete or resin. This method has been traditionally used by specialized goldfish breeders in major production regions (Yatomi, Aichi; Yamato-Koriyama, Nara; Edogawa Ward, Tokyo; etc.) and excels at selection, breeding, and mass rearing.

In recent years, small to medium-scale earth ponds and plastic tub cultivation using yards and balconies have been reassessed among hobbyist goldfish enthusiasts, and this method is gaining attention in the cultivation of large-sized ranchu and high-quality ornamental goldfish.

Benefits of Earth Pond and Earth Floor Cultivation

Natural Filtration and Ecosystem Formation

Abundant aerobic and anaerobic bacteria colonize the soil substrate, naturally performing not only the breakdown of ammonia and nitrite (nitrification) but also the denitrification of nitrate (anaerobic decomposition).

• Surface (aerobic layer): Nitrification of ammonia → nitrite → nitrate
• Deep layer (anaerobic layer): Denitrification of nitrate → nitrogen gas

In mature earth ponds where this system is functioning, water change frequency can be significantly reduced. Some professional goldfish breeders say that "mature earth ponds can be maintained with only a few water changes per year."

Natural Generation of Microorganisms and Live Food

The following microorganisms naturally develop in the soil and water, serving as live food for goldfish:

• Daphnia: High-protein live food that promotes growth in fry
• Copepods and midge larvae: Nutrient-rich organisms
• Green algae and diatoms: Phytoplankton that naturally develop in water (the basis of green water)

Especially during fry-rearing periods, cultivation in earth ponds promotes development through live food feeding, making it easier to raise individuals with good body shape and coloration.

Water Temperature Stability

Large earth ponds have high water volumes, providing strong buffering against temperature fluctuations and preventing sudden water temperature changes. Water temperature changes gradually with seasonal transitions, creating an environment where goldfish can easily acclimate.

How to Build an Earth Pond

Site Selection

• Sunlight: Partial shade with sunlight in the morning and shade in the afternoon is ideal (prevents excessive growth of blue-green algae and moss from excess direct sunlight)
• Drainage: A location where rainwater doesn't accumulate, or where drainage can be ensured
• Water source access: Access to tap water or well water for water changes

Pond Structure

Small-scale earth pond (for hobbyists)

• Volume: 200–1,000 L
• Depth: 30–60 cm (too shallow causes large water temperature fluctuations)
• Area: 1–4 m²

Large-scale earth pond (for breeders)

• Volume: 1,000–10,000 L or more
• Depth: 60–100 cm
• Area: 4–20 m² or more

Choosing Substrate

Substrate thickness should be approximately 5–15 cm. If too thick, anaerobic conditions become too strong, risking hydrogen sulfide generation.

Water Startup

Immediately after creating an earth pond, soil particles stir up easily and water becomes turbid.

1. Fill the pond with water: If using tap water, dechlorinate it (for large volumes, let stand for a few days or use a liquid dechlorinator)
2. Establish green water (blue water): Leave exposed to sunlight for 1–2 weeks to allow plankton to proliferate and form green water
3. Verify ammonia and nitrite: Always test with a kit before introducing fish (should be zero)
4. Introduce goldfish: Start with a small number and observe for 1–2 weeks before adding more

Management

Water Quality Management Approach

In mature earth ponds where natural filtration functions, water quality remains stable if stocking density is appropriate.

Basic Management Parameters

Stocking Density

Proper density management is essential to maximize the natural filtration capacity of earth ponds.

• Overstocking is strictly prohibited as it exceeds natural filtration capacity and causes rapid ammonia increase
• Guideline: 5–10 adult fish (15 cm) per square meter (depending on pond depth and maturity)
• Reduce density for large varieties such as ranchu

Green Water (Blue Water) Management

Moderate green water is beneficial in earth ponds, but excessive blue-green algae (such as Microcystis) is harmful.

Good green water: Even mixture of diatoms and green algae (bright green, transparent)

Bad blue-green algae: Excessive blue-green algae bloom (murky dark green, foam, odor)

Blue-green algae suppression strategies:

• Increase water change frequency
• Reduce direct sunlight by 30–50% with shade cloth
• Increase aeration (air pumping)

Overwintering Management

• When water temperature drops to 10°C or below, goldfish enter hibernation (semi-dormancy)
• Gradually reduce food based on water temperature; stop feeding below 8°C
• In regions where ponds freeze, use insulation covers (vinyl sheets or insulating material)
• The pond bottom must be deep enough for goldfish to hide (minimum 30 cm or more)

Precautions for Earth Pond Cultivation

Predator Control

Outdoor cultivation faces predation risks from wild birds (herons, kingfishers, crows) and mammals (cats, raccoon dogs).

• Install bird netting (20–40 mm mesh) over the entire pond
• Raise pond sides at least 50 cm above ground
• Cover at night

Mud Management

Over time, fish waste, uneaten food, and organic matter accumulate on the soil bottom.

• Annual or biannual bottom cleaning: Temporarily move goldfish to another container and remove part of the bottom mud (approximately 1/3 of the total)
• Complete mud removal is prohibited as it destroys beneficial bacteria
• Removed mud can be reused as plant fertilizer

Parasite and Disease Management

In earth ponds mimicking natural environments, parasites such as fish lice (Argulus) and anchor worms (Lernaea) can easily invade.

• Regular observation of fish (1–2 times per week)
• New fish must be quarantined for at least 2 weeks
• If fish lice are confirmed, perform early physical removal and medicated bath (such as Refish)

Why Breeders Use Earth Ponds

The reason specialized goldfish breeders insist on earth ponds is not merely tradition.

1. Cost-effectiveness: Low maintenance costs even for large-scale rearing
2. Easy-to-select body development: Natural feed and low-stress cultivation in a natural environment develop superior body shape
3. Natural coloration: Natural color enhancement from green water and live food
4. Breeding conditions: Seasonal changes (water temperature and light intensity) naturally cycle, making breeding induction easier

Even at home, you can experience the "true nature" of goldfish cultivation by trying small earth ponds or plastic tub rearing using Arakida soil. Please challenge yourself to try traditional earth pond cultivation.