Aquaponics in winter: keeping it running below freezing

Insulation, greenhouse heating, species that tolerate cold, and the math on whether heating the water is worth the electricity. Options for every climate.

Winter is where most outdoor aquaponics systems either prove their design or reveal their weaknesses. Water freezing in pipes, fish metabolism slowing to a crawl, nitrifying bacteria becoming sluggish, and plants dying from cold, these are all solvable problems, but they require planning before the first frost, not after.

The approach depends on your climate severity and what you're willing to spend on heating.

What cold does to the system

Fish metabolism

Fish are ectotherms. Below their comfortable temperature range, metabolism slows, feeding decreases, and waste production drops. Tilapia stop eating below about 18 C and die below 12 C, though blue tilapia, the most cold-tolerant species, hangs on a few degrees lower. Trout tolerate cold well, down to about 4 C, but grow very slowly below 10 C. Goldfish survive near-freezing temperatures but enter a dormant state, producing minimal waste.

Reduced feeding means reduced ammonia input, which means reduced nitrogen for the plants. In a cold system, the plants are nutrient-starved because the fish aren't producing enough waste, even if there are enough fish for summer operation.

Nitrifying bacteria

The bacteria that power the nitrogen cycle slow with temperature. Nitrification rises with warmth up to around 25-30 C; drop from 25 C to 15 C and the processing rate roughly halves. Below 10 C it is minimal, and near freezing the bacteria go dormant, alive but barely working.

This means ammonia from any remaining fish feeding isn't converted to nitrate efficiently. Ammonia can accumulate even at low feeding rates if the bacteria aren't active enough to process it. In cold-winter systems, reduce feeding dramatically (to 0.5-1% of body weight per day or less) and test ammonia weekly.

Pipes and plumbing

Water expands when it freezes. Frozen pipes crack, fittings split, and pump housings shatter. Any exposed plumbing, especially above-ground runs between the fish tank and grow beds, is vulnerable during a hard freeze.

Plants

Most warm-season crops (tomatoes, basil, peppers) die at or near freezing. Even cold-tolerant crops (lettuce, kale, spinach) stop growing below about 5 C and suffer tissue damage below -2 C. An unprotected grow bed in a freezing climate produces nothing during winter months.

Strategy 1: shut down for winter (cold climates)

If your system is outdoor, unprotected, and you experience months of temperatures below freezing, seasonal shutdown may be the most practical option. Harvest or relocate the fish to an indoor tank. Drain the system completely to prevent freeze damage. Clean and store the pump and air stones indoors. Cover grow beds to prevent them from filling with snow and ice.

Restart in spring by refilling, cycling the biofilter (it may need a partial re-cycle; test ammonia and nitrite to confirm the bacteria survived), and restocking with fish and plants once temperatures are consistently above 15 C.

This approach costs nothing in winter heating but produces no food for 3-5 months depending on climate. It's the default for systems in USDA zones 5 and colder without a greenhouse.

Strategy 2: insulate and heat the fish tank

Keep the fish alive and minimally fed through winter while accepting that plant production will be reduced or paused.

Insulate the fish tank. Wrap with rigid foam insulation (XPS or EPS, 5-10 cm thick). Insulated tanks lose heat far more slowly, reducing the energy needed to maintain temperature. A 1000-liter tank insulated with 5 cm of XPS in a 0 C ambient environment loses roughly 40-60% less heat than an uninsulated tank.

Add a tank heater. An aquarium heater (300-500W for a 500-1000 liter tank) maintains water temperature in the fish's survival range. The electricity cost depends on ambient temperature and insulation quality. In a well-insulated tank at 0 C ambient, maintaining 15 C water temperature for tilapia costs roughly $30-60/month in electricity at $0.15/kWh. For trout, you only need to prevent freezing, which costs much less.

Protect plumbing. Insulate all exposed pipes with foam pipe insulation. For runs that can't be insulated adequately, drain them between pump cycles (this requires a timer-controlled pump rather than continuous operation). Heat tape on critical plumbing runs prevents freezing in the most vulnerable sections.

Reduce or stop the grow beds. If plant production isn't viable, you can circulate water through the biofilter only (to maintain the bacteria) and bypass the grow beds entirely. This simplifies winter operation and reduces heat loss from the larger surface area of the grow beds.

Strategy 3: greenhouse

A greenhouse extends the growing season by trapping solar heat and providing a controlled environment for both fish and plants. This is the approach used by most year-round aquaponics operations in temperate climates.

Sizing and orientation. A greenhouse sized to enclose the entire system (fish tank, grow beds, and sump) is most efficient. Orient the long axis east-west to maximize solar gain in winter. A south-facing glazed wall captures the most winter sunlight (in the northern hemisphere).

Glazing. Double-wall polycarbonate is the standard for aquaponics greenhouses because it insulates better than single-pane glass (R-value of 1.5-2.0 vs 0.9 for single glass) and is impact-resistant. Triple-wall polycarbonate costs more but insulates even better (R-value 2.0-2.5).

Supplemental heating. Even a well-insulated greenhouse needs heating during cold nights and overcast periods. Options include propane heaters, electric space heaters, wood stoves, and ground-source heat pumps. For a small greenhouse (3 x 6 meters), a 30,000 BTU propane heater maintains adequate temperature during moderate freezing conditions. For sustained deep cold (below -15 C), more heating capacity is needed.

The fish tank itself acts as a thermal mass, absorbing solar heat during the day and releasing it slowly at night. This moderates temperature swings inside the greenhouse. Larger water volumes provide more thermal mass.

Cost. A basic DIY greenhouse (PVC pipe frame, double-layer polyethylene film) costs $200-500 for a 3 x 6 meter structure. A polycarbonate greenhouse of similar size costs $800-2000 for a kit. Heating costs during winter months add $30-100/month depending on climate and insulation quality.

Strategy 4: move indoors

For small systems, moving the fish tank indoors for winter is the simplest approach. A 100-200 liter fish tank with an air stone fits in a basement, spare room, or heated garage. The fish stay warm at room temperature with no additional heating. Run a small grow bed on a shelf above the tank with a clip-on LED light.

This approach works for goldfish and small ornamental systems. It doesn't work well for production-scale tilapia systems that require 500+ liters of water and dedicated grow bed space.

Choosing cold-tolerant species

If winter operation without heating is your goal, choose fish and plants that thrive in cold:

Fish: Rainbow trout (peak growth around 12-16 C, tolerates cold water down to about 4 C). Goldfish (16-23 C optimal, survives near-freezing). Yellow perch (best around 16-22 C, tolerates down to 4 C). Channel catfish (24-29 C optimal, tolerates down to about 10 C but barely feeds below 13-15 C).

Plants: Lettuce, kale, spinach, Swiss chard, cabbage, broccoli, cauliflower, radish, and herbs like cilantro and parsley all grow in cool conditions (8-18 C). They won't grow fast, but they'll produce through the shoulder seasons and during mild winter days inside a greenhouse.

The running cost calculator can help you model the heating costs for different winter strategies and compare them to the value of year-round production.

Case study: zone 6 greenhouse aquaponics

A hobbyist in zone 6 (winter lows of -15 to -20 C) runs a 3 x 6 meter greenhouse with twin-wall polycarbonate glazing, a 500-liter fish tank with 20 tilapia, and two 1 x 2 meter raft beds growing lettuce and herbs.

September-October: The system operates normally with ambient greenhouse temperatures staying above 15 C during the day. The fish tank (500 liters of thermal mass) moderates overnight temperature drops. No supplemental heating needed yet. Cool-season crops planted in the rafts are growing well.

November: First frost. Overnight greenhouse temperatures drop to 5-8 C. The propane heater (30,000 BTU) kicks in when the thermostat hits 12 C. The fish tank temperature stays at 18-20 C because the water mass is slow to cool. Tilapia are still eating but at reduced rates. Monthly propane cost: $30-40.

December-January: Deep winter. Overnight lows outside reach -15 C. The heater runs most of the night, maintaining greenhouse air temperature at 12-15 C. The fish tank, insulated with rigid foam on three sides, holds at 16-18 C. Tilapia eat minimally at these temperatures. Lettuce and herbs grow slowly but steadily. Monthly propane cost: $60-90.

February-March: Days get longer. Solar gain increases. The heater runs less. By mid-March, the greenhouse is warm enough during the day that the heater only runs overnight. Fish feeding increases as water temperature rises. Spring crops are transplanted into the rafts.

Total winter heating cost (October through March): $250-400 in propane. The system produces lettuce, herbs, and greens throughout the winter, plus a batch of tilapia ready for harvest by late spring. Without the greenhouse, the system would have been shut down for 5 months and the fish relocated or lost.

Whether this pencils out financially depends on your valuation of winter-fresh produce and your fish. For many growers, the food security and continuity of the system justify the heating cost. For others, seasonal shutdown and restocking in spring is the more practical approach.

The running cost calculator can model your specific greenhouse heating costs based on your climate zone and fuel type.