Overwintering Koi in Basement Tanks: Feeding, Plants & Water Quality

Setup Essentials

Minnesota winter: 100-gal basement tank, 60°F water, 18 hatchlings (2") + 18 adults (6–8"). Pump + filter running. Darkness at night, some daylight.

Feeding Protocol

• Feed daily (water >50°F = active metabolism).
• High-quality sinking pellets (wheat-germ base) — easier to monitor consumption.
• Supplement: Blanched zucchini, peas, spinach (clip to side). Remove uneaten in 1 hour.
• No floating food — fouls surface, harder to track.

Plants & Light

Pond plants (water hyacinth, lettuce, anacharis) help absorb nitrates but need 10–12 hrs light. Add a shop light (6500K LED) on timer. "Hogwart" plants (likely hornwort) are excellent oxygenators.

Critical Maintenance

1. Test ammonia/nitrite 2×/week.
2. 25% water changes weekly (dechlorinated, temp-matched).
3. Salt at 0.1% (1 lb/100 gal) for stress reduction.
4. Cover tank (reduces evaporation, jump risk).

Stocking Check

36 fish in 100 gal is very heavy. Consider splitting into two 100-gal tanks or upgrading to a 300-gal stock tank. Overcrowding = disease outbreak.