Why Winterizing Matters
Koi are remarkably hardy fish — they can survive freezing temperatures as long as their pond doesn't freeze solid and their basic needs are met. But neglecting winter prep is one of the most common causes of koi loss. A little preparation in fall goes a long way toward bringing healthy fish through to spring.
Step 1: Monitor Water Temperature
Everything in winter koi care revolves around water temperature. Get a reliable pond thermometer and check it regularly as fall progresses.
- Below 65°F: Begin transitioning to cool season food
- Below 55°F: Reduce feeding to every 2–3 days
- Below 50°F: Stop feeding entirely
Koi cannot digest food in cold water. Undigested food in their gut can cause serious health problems or death over winter.
Step 2: Switch to Cool Season Food
Before you stop feeding entirely, transition your koi to a cool season formula like Blackwater Cool Season Koi Food. The low-fat, wheat-based formula is easy to digest as metabolism slows. Stop feeding completely once water temps drop below 50°F.
Step 3: Clean the Pond
Before winter sets in, remove as much organic debris as possible — fallen leaves, uneaten food, and waste. Decomposing organic matter consumes oxygen and produces harmful gases under ice. Use a pond net to skim the surface and a pond vacuum if needed for the bottom.
Step 4: Manage Your Filter
This is where many koi keepers make mistakes. Your biological filter contains beneficial bacteria that keep ammonia in check — but those bacteria also slow down in cold water.
- Keep your filter running through winter to maintain water circulation and gas exchange
- Do not clean your filter aggressively in fall — you'll destroy the bacterial colony you need
- If your filter is at risk of freezing, insulate it or move the pump intake deeper in the pond
Step 5: Maintain Oxygenation & Prevent Full Freeze
If your pond freezes completely, toxic gases from decomposing matter can build up under the ice and suffocate your fish. Prevent this by:
- Running a pond aerator or air pump to keep a hole open in the ice
- Using a pond de-icer or heater to maintain an opening — especially in climates with prolonged freezes
- Never break ice by force — the shockwave can stun or kill koi
Step 6: Reduce Pond Depth Concerns
Koi will naturally move to the deepest, warmest part of your pond in winter and enter a torpor-like state. They need at least 3–4 feet of depth to find water that stays above freezing. If your pond is shallow, consider a pond heater to maintain a safe temperature zone.
Step 7: Spring Restart
As water temps rise above 50°F in spring, your koi will become active again. Restart feeding gradually with cool season food, test your water parameters, and do a partial water change to refresh the pond after winter. Once temps are consistently above 65°F, transition back to your regular growth formula.
Have questions about winter koi care? Contact us — we're happy to help.