Pond Sizing
Minimum 1,000 gallons for koi. They reach 24+ inches and produce massive waste.
For goldfish: 200 gallons minimum, 500+ for a small school.
Depth: 4 feet minimum to overwinter koi (prevents complete freeze and gives temperature refuge).
Filtration
Pond filtration: minimum 2× pond volume per hour through filter.
Mechanical: sieve filter or vortex chamber removes solid waste.
Biological: bio-balls or matala mat hosts bacteria.
UV sterilizer: kills algae and parasites in suspension.
Stocking Density
Koi: 1 fish per 500 gallons (allowing for adult size).
Goldfish: 1 fish per 50 gallons.
Mosquito fish: 5–10 per 50 gallons (control breeding).
Overstocked ponds suffer green water blooms, parasite outbreaks, and stunted growth.
Koi
Origin: Japan (cultivated from common carp). 24–36 inch adults, 30+ year lifespan.
Varieties: Kohaku (white-red), Sanke (white-red-black), Showa (black-red-white), Asagi (blue-back), Ogon (gold).
Diet: floating koi pellets, supplemental fruits and vegetables (watermelon, lettuce).
Cost: $20 baby to $20,000+ for show-quality adults.
Goldfish
Best pond goldfish: comets, shubunkins, common goldfish. Reach 12–14 inches in ponds.
Avoid fancy goldfish (orandas, ranchus) outdoors — they cannot escape predators or compete for food.
Lifespan in pond: 15–20 years.
Mosquito Fish (Gambusia)
Tiny native fish that eat mosquito larvae. Excellent for backyard ponds.
Self-sustaining population. Tolerates a wide range of conditions.
In some regions, considered invasive — check local regulations.
Seasonal Care
Spring: gradual feeding resumption as water warms past 50°F. Test parameters; spring ammonia spikes are common.
Summer: feed 2× daily, monitor oxygen during heat waves (add air pump if surface is calm).
Fall: reduce feeding as water cools below 60°F. Stop entirely below 50°F.
Winter: minimal activity. Maintain a hole in any ice cover (use heater or air pump). Don't feed.