Pond Care & Water Quality

Koi tolerate a wide pH range but sudden swings are dangerous. pH naturally rises through the day as plants consume CO₂ during photosynthesis and drops overnight. A swing of more than 0.5 in 24 hours is stressful. Crushed coral or oyster shell in the filter stabilises pH in soft-water ponds.

Produced by fish waste, decomposing food, and dead plant matter. Highly toxic in its unionised form (NH₃), which increases with rising pH and temperature. New ponds spike during the nitrogen cycle. Test weekly; daily if you see lethargic fish or red streaking on fins.

The intermediate stage of the nitrogen cycle. Converts to less harmful nitrate once the filter is fully established (typically 4–8 weeks on a new pond). Nitrite interferes with oxygen transport in blood. Adding pond salt at 0.3% temporarily blocks nitrite absorption during spikes.

The safe end-product of the nitrogen cycle. Accumulates over time — controlled by regular partial water changes (10–20% per week) and plant uptake. High nitrate indicates insufficient water changes or overstocking.

Koi gasp at the surface when oxygen drops. Highest risk in summer (warm water holds less O₂) and overnight (plants consume O₂ in darkness). Aeration — waterfall, air stones, or venturi — should run 24/7. Never turn off aeration at night.

KH buffers pH — it prevents pH crashes. A KH below 50 ppm means your pH is unstable and a crash is possible after heavy rain or a large water change. Raise KH with sodium bicarbonate (baking soda) or potassium bicarbonate. Address this before adjusting pH directly.

Spring

• —Restart filtration 2–3 weeks before resuming feeding so bacteria re-establish
• —Test all parameters — winter sludge may have spiked ammonia and nitrite
• —Partial water change (20–30%) to flush accumulated nitrate
• —Inspect pump and UV clarifier; clean or replace UV bulb (bulbs degrade after 9 months of use even if still lit)
• —Net pond to remove leaf litter before it decomposes

Summer

• —Test water weekly — high temperatures accelerate ammonia toxicity
• —Increase aeration; add a second air stone if water is above 27°C
• —Perform 10–15% water changes weekly or bi-weekly to control nitrate
• —Remove dying algae before it crashes and creates an ammonia spike
• —Monitor fish closely — heat stress and parasites both peak in summer

Autumn

• —Net pond to catch falling leaves before they sink
• —Reduce feeding as temperatures fall below 15°C; switch to wheat germ
• —Final partial water change before koi enter winter slowdown
• —Service filter media — do not clean everything at once or you'll lose your bacterial colony

Winter

• —Stop feeding below 4.5°C (40°F)
• —Keep a hole in ice using a pond heater or bubbler — never break ice by force (shockwave stresses fish)
• —Run pump at lowest setting or switch to a separate small pump near the surface to avoid disturbing the warmer water layer at the bottom where koi rest
• —Do not run waterfall if temperatures will drop below freezing — ice forming on rocks can crack liner and block flow