How to Acclimate New Fish in Cold Weather — And When It’s Too Late to Add Fish to Your Pond

by Toledo Goldfish Admin December 12, 2025

koi pond that has fish in it and covered in snow on the banks
As temperatures drop and pond season winds down, many pond owners still look to add new life to their water features. Whether you’re introducing koi, butterfly koi, goldfish, crayfish, or minnows, cold-weather acclimation becomes especially important for ensuring your new arrivals transition safely into their new home.

At Toledo Goldfish, we ship fish year-round as long as weather conditions allow—but proper acclimation is essential, particularly in late fall and early winter.

Why Cold-Weather Acclimation Matters

Cooler temperatures slow a fish’s metabolism and reduce oxygen demand, but they also make fish more sensitive to sudden environmental changes. A rapid shift in temperature or water chemistry can cause shock, stress, or even death. Taking a little extra time during acclimation dramatically increases the health and safety of your new pond additions.

Step-by-Step Guide: Acclimating Fish When Temperatures Are Colder

Along with our normal acclimation tips, here are a few more things to think about in colder temperatures.

1. Float the Bags Longer Than You Would in Summer

In warm weather, floating for 15–20 minutes is usually enough. In cold weather, extend this to 20–30 minutes, if the water temperatures in the bag and new environment are very different. This helps gradually bring the fish’s bag water temperature closer to your pond’s temperature—crucial when outdoor water is significantly cooler.

2. Avoid Mixing Shipping Water Directly Into Your Pond

During cold months, fish have a lower tolerance for sudden shifts in water chemistry. We don't add anything extra to the water, but during shipping the fish will have waste and that waste can have a high ammonia level which can negatively affect your fish once in their new home.

Instead of dumping the bag:

Gently net the fish out,
Then release them into the pond,
Discard the shipping water.
This is especially important in smaller water systems, in larger ponds you don't have to worry about this as much.

3. Add Darkness and Reduce Stress

Cold-weather shipping can stress fish, so reduce extra stimulation:

Turn off waterfalls or fountains temporarily if the water is splashing heavily.
Release fish during a calm part of the day.
Keep pets and movement around the pond minimal during acclimation.

4. Let Fish Sink Naturally

New arrivals will often head straight to the bottom to adjust.
This is normal—especially for koi, butterfly koi, and goldfish—who prefer deeper, stable temperatures when it’s cooler.

5. Do Not Feed Immediately

Fish won’t be ready to eat right away in cooler weather.
Wait 24–48 hours, and do not feed at all if your pond water is already below 50°F.

Cold-Weather Tips by Species

⭐ Koi & Butterfly Koi

Hardy in cool water but sensitive to sudden temperature swings.
Acclimate slowly, and avoid introducing koi if the pond is near freezing.

⭐ Goldfish

Extremely cold-tolerant and hardy.
Great for late-season stocking, but not once ice begins forming.

⭐ Minnows (Rosie Reds and Black Fatheads)

Adapt easily to cold water.
Still require gentle acclimation to prevent temperature shock.

⭐ Crayfish

Prefer stable temperatures and can become sluggish in very cold ponds.
Avoid adding crayfish once water temperatures drop into the mid-40s or below.

⭐ Trapdoor Snails

Prefer water temperatures and can become sluggish in very cold ponds.
While they can survive just fine in cold water year round and they like to dig down in the warmer parts of the pond bottom during cold periods.
It is common for snails to take a long time to open up and move around in colder water. As long as their "trapdoor/foot" is attached they are most likely alive!

When Is It Too Late to Add New Fish to Your Pond?

While fish can tolerate cooler conditions, there is a cutoff point in the year when introducing new pond life becomes unsafe.

General Rule of Thumb

Do NOT add new fish when pond water is consistently below 45°F.
Never add fish if nighttime temperatures are cold enough for thin ice to form.

Ideal Stocking Window

Spring through early fall (when water temps are 55–75°F).
Late fall additions can be safe as long as temperatures remain stable and you acclimate slowly.

Why You Should Avoid Very Late-Season Stocking

Fish cannot adjust quickly enough to dramatic temperature drops.
Reduced immune response in cold water increases stress.
Limited time to build up energy reserves before winter dormancy.

At Toledo Goldfish, we monitor weather conditions closely and may hold shipments if temperatures drop too low to ensure your fish arrive safe and healthy.

Final Takeaway

Cold-weather stocking can be successful—as long as you acclimate slowly and pay attention to pond temperature. If your pond water is still above 50°F and weather is stable, koi, goldfish, minnows, and even crayfish can still thrive with proper care.

But once temperatures dip too low, it’s best to hold off until spring. Your fish will thank you for the warm, steady start next season!