Long Island falls in USDA Hardiness Zone 7, where winter temperatures regularly drop into the teens and single digits. Your koi pond needs specific preparation to keep your fish safe, your equipment intact, and your water feature ready to come back to life in spring. Follow this checklist every fall — typically starting in October when water temperatures begin dropping toward 50°F.
Not comfortable doing it yourself? We offer professional fall pond closing service throughout Nassau County. Book your winterization below.
Stretch a fine-mesh pond net across the water surface before trees begin dropping leaves. Decomposing leaves release ammonia and deplete oxygen — the two biggest threats to overwintering fish. Remove the net and leaves weekly, or install a permanent net frame that spans the full pond.
When water temperatures drop below 50°F, koi's metabolism slows dramatically. Switch from your regular high-protein food to a wheat-germ-based cold-water formula — it's easily digestible at low temperatures and won't decompose in the pond if uneaten. Once temperatures drop below 40°F, stop feeding entirely. On Long Island this transition typically happens in late October through November.
Your main recirculating pump should be removed and stored indoors before the first hard freeze. Running a pump in near-freezing water can damage the motor and, more importantly, pulling cold surface water down into the deeper warmer zones where fish are resting can fatally chill them. Clean the pump before storage and inspect the impeller.
Note: Pondless waterfalls use a different setup — consult your installer on whether to run or store the pump over winter.
Replace your main pump with a small winter aerator (air pump with an air stone). Run it near the surface — not at the bottom — to maintain a gas exchange point without disturbing the warmer water at the pond floor where fish congregate. The aerator keeps a small area from freezing solid and prevents toxic gas buildup under the ice. This is non-negotiable for ponds with fish on Long Island.
A floating pond de-icer (also called a pond heater) doesn't heat the water — it simply maintains a small hole in the ice to allow toxic gases to escape and oxygen to enter. In Long Island winters, the aerator and de-icer work as a team. The de-icer ensures the gas exchange hole stays open during the coldest nights, even when temperatures plunge below zero.
Apply a cold-water beneficial bacteria treatment in fall before temperatures drop too far. Cold-water bacteria formulas remain active down to about 35°F and continue breaking down organic waste (leaves, fish waste) that would otherwise accumulate and spike ammonia levels under the ice. A clean pond going into winter is a healthy pond coming out in spring.
Cut back marginal plants and remove any tropical aquatics before they rot in the cold water. Hardy water lilies can be left in place if your pond is deep enough — they'll go dormant and return in spring. Remove dead plant matter from the pond to reduce decomposing organics during winter.
Long Island's USDA Zone 7 designation means average minimum winter temperatures between 0°F and 10°F. In practice, Nassau County sees prolonged cold stretches where pond surfaces freeze solid for weeks at a time. Koi can survive this — they're remarkably cold-hardy — but only if the pond is properly prepared and the water remains oxygenated throughout winter.
The minimum safe depth for overwintering koi on Long Island is 18–24 inches. At this depth, the water near the bottom stays above freezing even when the surface is iced over. If your pond is shallower, consider adding a submersible pond heater to maintain a safe temperature zone.
Spring reopening — removing the aerator, reinstalling the pump, and beginning feeding again — happens when water temperatures consistently stay above 50°F, typically late March to April on Long Island.
Book your fall pond winterization before our schedule fills — Nassau County service available throughout October and November.