When your pond freezes over, the danger to your fish isn't the cold. It's the air.
Pond fish like koi and goldfish handle cold water just fine. In winter they slow down and rest near the bottom, where the water stays warmest. The real risk is a solid sheet of ice. Leaves, fish waste, and other debris keep breaking down all winter, releasing gases that need somewhere to go. When the surface is sealed, those gases build up under the ice and fresh oxygen can't get in.
The fix is simple: keep one small hole open in the ice so your pond can breathe until spring. The right equipment for that depends on how cold your winters get and how big your pond is, and this page will help you choose in about a minute.
Find the right setup for your pond
Questions before you buy? Call (512) 601-8197 and talk to a real person.
How keeping a hole open works
- Lets harmful gases escape. Gases from decaying debris vent out through the opening instead of building up under the ice.
- Lets oxygen in. Air exchange at the opening keeps oxygen available for your fish all winter.
- Runs on its own. Set up your aerator or de-icer before the first hard freeze and let it run until the ice is gone.
Winter mistakes to avoid
- Don't smash the ice. Hitting the ice sends shock waves through the water that can stress or harm resting fish. If you need to open the ice, melt a hole with hot water instead.
- Don't leave the air stone at the deepest point. In winter, raise it to a shelf about a foot below the surface so it doesn't mix away the warmer water at the bottom where your fish rest.
- Don't count on your waterfall. Waterfalls and streams can freeze and divert water out of the pond. Aeration is the safer way to keep water moving in winter.
- Don't buy a bigger heater than you need. A de-icer that's too large for your climate just costs more to run. Use the sizing guide below.
Step 1: Aerator or de-icer?
It depends on how cold your winters get. Find your USDA hardiness zone by entering your ZIP code on the USDA Plant Hardiness Zone Map.
Zones 6, 7 or 8: milder winters
An aeration kit usually keeps a hole open on its own, and it costs pennies a month to run. It also adds oxygen to the water year-round.
Zones 3, 4 or 5: long, hard freezes
Bubbles alone can freeze over during deep cold snaps. A de-icer is the more dependable choice. For the best protection, many pond owners in cold zones run a de-icer and an aeration kit together.
No fish, just plants? Hardy pond plants usually come through winter fine under ice, so you may not need any winter equipment.
Step 2: Choose your aeration kit by pond size
Not sure how many gallons your pond holds? Multiply length × width × average depth (in feet) × 7.5. For example, a pond 10 ft long, 6 ft wide, and 2 ft deep on average holds about 900 gallons.
Small ponds: up to 1,000 gallons
Typical backyard and patio ponds up to 2 ft deep.
Airmax PondAir 2 · $74.99 · about $0.20/month to run
Medium ponds: up to 2,000 gallons
Larger garden ponds up to 2 ft deep.
Airmax PondAir 4 · $109.99 · about $0.40/month to run
Large ponds: 2,000 to 8,000 gallons
Koi ponds and water gardens up to 4 ft deep.
Airmax KoiAir 1 · $299.99 · about $0.85/month to run
Pond over 8,000 gallons? Call (512) 601-8197 and we'll help you size it right. Running costs assume the kit runs 24/7.
Step 2: Choose your de-icer size
K&H Thermo-Pond Perfect Climate De-Icer
Floats or sinks, and a built-in thermostat runs it only when the water nears freezing. Safe for all pond liners. Choose the smallest wattage your zone and pond size call for:
| Zone | Under 100 gal | 100–400 gal | 400–750 gal | 750–1,000 gal | 1,000–1,200 gal |
|---|---|---|---|---|---|
| 3 | 250W | 750W | 750W | 1500W | 1500W |
| 4 | 250W | 750W | 1500W | 1500W | 1500W |
| 5 | 250W | 250W | 750W | 750W | 1500W |
250W $59.99 · 750W $69.99 · 1500W $79.99
Maximum running cost if it ran nonstop: about $12.50, $37.50, or $75 a month. The thermostat means most owners pay less. Ponds over 1,200 gallons need more than one de-icer.
Why buy from Pondwell
- Help choosing the right setup. Call (512) 601-8197 or email contact@pondwell.com and we'll help you pick the right size for your pond.
- Free shipping on orders over $49. Orders ship within 1–5 business days. Shipping details
- 60-day returns. Return unused items within 60 days of delivery. Return policy
Not ready to buy yet? Get our free Winter Pond Checklist
Everything to do before the first hard freeze, from leaf cleanup and fish feeding to equipment prep, on one simple page. We'll email it to you, along with occasional seasonal pond tips.
No spam. Unsubscribe anytime.
Common questions
When should I set up my aerator or de-icer?
Before the first hard freeze. It's much easier to place equipment in open water than to work around ice.
Do I need to keep my whole pond from freezing?
No, and you don't need to. The goal is a vent hole so gases can escape and oxygen can get in.
Can I use an aerator and a de-icer together?
Yes. In cold zones this is a common setup: the de-icer keeps the hole open through hard freezes, and the aerator adds oxygen.
Can my fish stay in the pond all winter?
Hardy pond fish like koi and goldfish usually overwinter well in ponds deep enough that they don't freeze solid, generally at least 18 to 24 inches deep in most areas, and deeper in colder climates.
Can I leave it running all winter?
Yes. Aeration kits are designed to run continuously, and the de-icer's thermostat turns it on and off on its own. Keep air pumps in a dry, sheltered spot out of rain and snow.
Get your pond ready before the freeze
Find the right setup for your pond
Questions? Call (512) 601-8197 or email contact@pondwell.com.