Pond Winterization in New Jersey: The Do's and Don'ts

Koi are cold-blooded, and their digestive systems slow down dramatically as water cools. The problem is that their instinct to come to the surface when you approach does not. They will still beg for food at 45 degrees. They will still take a pellet from your hand. And they cannot digest it at all.

Undigested food sitting in a koi's gut through a cold New Jersey winter can cause bacterial infections that kill fish in January or February with no visible warning. The fish that seemed fine going into winter suddenly are not.

• Switch to a wheat germ cold water formula when water temperatures drop below 65 degrees F
• Feed every other day or less between 55 and 65 degrees F, only what fish finish in under 2 minutes
• Stop feeding entirely once water temperatures are consistently below 50 degrees F
• Use a pond thermometer, not the calendar or the air temperature — water temperature lags behind air temperature by days

This is the single most consequential pond winterization decision a New Jersey pond owner makes, and the most commonly misunderstood. Your pond needs to breathe all winter. Not circulate heavily, not filter aggressively, but breathe.

Even in a dormant winter pond, biological activity is still happening slowly. Fish produce small amounts of waste. Organic matter decomposes. Those processes release gases like ammonia, carbon dioxide, and hydrogen sulfide that in an open pond simply escape into the air. In a pond that is completely frozen over, those gases have nowhere to go. They concentrate under the ice all winter and can reach lethal levels by February. This is called winter kill, and it is entirely preventable.

• Install a floating de-icer before the first hard freeze to maintain a small opening in the ice surface
• Add an aerator with the diffuser positioned near the surface, not at the bottom — you want gas exchange, not disruption of the warm bottom layer where fish rest
• Check that the opening in the ice is staying clear during sustained cold snaps

• Do not run your waterfall in freezing temperatures — it acts like a radiator, pulling heat out of the water and can cause ice dams and pump failures
• Do not turn off all equipment and check out until spring — minimal active management is fine, zero management is not

Water expands when it freezes. Anything that holds standing water and is left outdoors through a New Jersey winter is at risk, and the repairs are rarely cheap. Equipment protection is the part of pond winterization most people think of last, and it should be one of the first.

• Remove UV clarifiers from the pond, drain them completely, and store them indoors — the quartz sleeve cracks in a single freeze and replacements are expensive
• Drain any external plumbing, fittings, or valves that can hold standing water before freezing temperatures arrive
• If you are running a pump through winter, confirm it cannot ice over and check it periodically

Pond winterization in New Jersey happens in the middle of leaf season, and that is not a coincidence. Leaves that make it into your pond become the organic matter that decomposes all winter, producing the gases your de-icer is working to release. The less organic matter in your pond going into winter, the less work the season asks of your system.

• Net the pond before more leaves drop — a raised frame keeps leaves from sagging into the water
• Remove any leaf debris already at the bottom using a long-handled pond net or vacuum
• Trim back marginal plants like iris and cattails to just above the waterline before their dying foliage falls in
• Move hardy lily tubers to the deepest section of the pond so they stay below the freeze line
• Remove tropical plants entirely — water hyacinth, water lettuce, and banana plants will not survive a New Jersey winter

A gentle partial water change of 15 to 20 percent using dechlorinated water in early October lowers the overall organic load heading into winter and gives your fish a cleaner environment to slow down in. Low effort, genuinely useful.

Your regular pond bacteria becomes less effective as water temperatures drop. Cold water formulas are specifically designed to stay active in cooler conditions and keep breaking down organic waste during the temperature transition. That matters because the cleaner your water chemistry is going into winter, the less toxic gas buildup happens under the ice.

• Switch to a cold water beneficial bacteria formula in mid-fall when temperatures consistently dip below 55 degrees F
• Dose weekly through October and into November as long as water temperatures stay above 40 degrees F
• Use in conjunction with your de-icer, not instead of it — bacteria and gas exchange work together

This is one of the lower-effort, higher-payoff steps in New Jersey pond winterization. It costs very little and can make a real difference in water quality when spring arrives.