When a pond freezes over, the fish underneath are cut off from fresh oxygen, and gases from decaying leaves and muck have nowhere to go. Keeping part of the surface open fixes that. The two tools for the job are a de-icer and a winter-set aerator, and they work in different ways. This guide explains how each one works, when to use which, and what to do if your pond almost never freezes.
The short version
- A de-icer is a small motor and propeller that pushes slightly warmer bottom water up to the surface to keep an opening in the ice. Paired with a thermostat controller, it only runs when it needs to.
- A diffused aerator can do the same job in winter if you move the diffuser to shallow water, and it keeps adding oxygen as well.
- You only need a small opening. Kasco says just 1 to 2 percent of the surface needs to stay open, and Iowa State Extension gives about 2 percent as a guide.
- In south Louisiana and similar climates, hard freezes are short. Most ponds need neither, but an aerator already in the pond should keep running.
Why open water matters in winter
Winter fish kills, often called winterkill, happen when ice and snow block sunlight. Plants and algae stop producing oxygen, but decaying organic matter on the bottom keeps using it up. Penn State Extension describes this as the main cause of winter fish kills in ponds. An opening in the ice lets the pond exchange gases with the air, and in a pond with an aerator it lets oxygen keep coming in.
How a pond de-icer works
In winter, the warmest water in a pond sits on the bottom. A de-icer, like the Kasco De-Icer, is a submersible motor and propeller that pushes that warmer water up toward the surface. Kasco says this upward flow is what creates the opening in the ice. Kasco de-icers come in 1/2, 3/4 and 1 HP, need at least 4 feet of water, and carry a 3-year warranty.
De-icers are also widely used around docks and boat lifts, where they keep ice from forming against pilings and hulls.
Use a controller. Running a de-icer around the clock wastes electricity on days the pond would not freeze anyway. Kasco's C-20 time and temperature controller turns the unit on only below a temperature you set and can limit it to certain hours. The C-10 version uses temperature only.
Using an aerator in winter
If you already have a diffused aeration system, you can usually use it to keep a hole open instead of buying a de-icer. Two changes matter:
- Move the diffuser shallower. In summer, diffusers sit in the deepest part of the pond to mix the whole water column. In winter, Ohio State Extension recommends shifting the diffuser to a shallow area, and suggests placing an air stone about 2 feet below the surface to keep a hole open.
- Run fewer diffusers. Kasco recommends fewer diffusers and moving them toward shore in winter.
The reason is supercooling. Circulating the deep, warmer water to the surface all winter can chill the entire water column and stress fish, which is exactly what you are trying to avoid. A shallow diffuser keeps the surface moving while leaving the warmer bottom layer alone.
Kasco also recommends avoiding splashing devices in winter. Fountains and surface aerators throw water into cold air, which cools the pond and can build ice on the unit. Pull or turn off fountains in freezing climates and remove fountain lights before ice forms.
De-icer vs aerator: which to use
| De-icer | Diffused aerator (winter setup) | |
|---|---|---|
| Keeps an opening in the ice | Yes | Yes, with the diffuser in shallow water |
| Adds oxygen | Indirectly, through the opening | Yes |
| Protects docks and boat lifts | Yes | Not really |
| Runs only when needed | Yes, with a C-10 or C-20 controller | Usually on a timer or run continuously |
| Best for | Ponds without aeration, docks, marinas | Ponds that already have a diffused system |
If you already aerate, use your aerator. If you do not, or you need to protect a dock, a de-icer with a controller is the simplest answer.
Do you need either one in Louisiana?
For most ponds along the Gulf Coast, the honest answer is no. Freezes are short and rarely put a lasting sheet of ice on a pond. Kentucky State University notes that in southern climates, diffusers are sometimes pulled and cleaned in late fall and reinstalled before spring stratification. If your system already runs year-round, keep it running through a cold snap, because a thin skin of ice still slows gas exchange. Farther north, where ponds stay frozen for weeks, plan on a de-icer or a winter-set aerator.
A safety warning about ice
Any pond with a de-icer or aerator running will have thin, weak ice around the opening, and the edge of that opening moves as conditions change. Ohio State Extension says aerated ice is not likely to be safe enough to support weight, and Penn State puts it more simply: no ice is completely safe. Keep people, pets and vehicles off the ice on any aerated pond and post signs if others use the property.
Frequently asked questions
Should I run my aerator all winter?
Not necessarily. Penn State Extension says the system does not need to run continuously all winter and suggests turning it on when ice forms and off when the pond is ice-free. If you do run it, use the shallow, reduced setup described above.
How big an opening do I need?
Small. Kasco says 1 to 2 percent of the surface area, and Iowa State Extension uses about 2 percent. A de-icer sized for your pond and depth will usually do this easily.
Can I just clear the snow off instead?
Clearing snow lets sunlight reach the plants so they keep making oxygen. Penn State suggests keeping about 30 percent of the ice clear of snow as an alternative. Ohio State advises choosing either aeration or snow removal, not both, because aerated ice is not safe to walk on.
Will a de-icer hurt my fish?
No. It moves a relatively small amount of water toward the surface. Problems come from mixing the whole pond, which is why aerators get moved shallow in winter.
Need help with winter setup?
Tell us your pond size, depth, and whether you already have aeration, and we will recommend the right de-icer or show you how to set up your aerator for winter. Call 1.888.775.2402, browse pond de-icers, or read our pond aeration guide.