Your favorite pond shop is closing down its fish systems for the season, and they’re running fantastic end-of-season sales on koi and goldfish you’d love to bring home. It’s late in the year, though, and adding them into your pond doesn’t feel quite right. Still, they’d make great additions come spring. Rather than walking out empty-handed, consider these ideas for enjoying those beautiful bargain fish indoors this winter and giving them a strong start when pond season returns.
Bringing Fish Indoors for Winter
Some pond owners bring all their fish inside during the winter to protect them from predators and harsh weather—and continue to enjoy their brilliant colors during an otherwise dull time of year. Depending on your pond size and budget, you might already be doing this. If so, that’s awesome. Hopefully this article offers a few new ideas or considerations. But if you’ve assumed keeping fish indoors was impossible on a small scale, I’m here to tell you it’s not. You’ve got options.
The best indoor location will depend on what you have access to. At a minimum, you’ll need electricity, above-freezing temperatures, and access to water for performing water changes. Common wintering areas include heated garages, basements or enclosed porches.
Keep in mind: the warmer the water, the faster your fish will grow—and the more frequently you’ll need to manage waste through water changes. No matter your setup, overwintering indoors also offers a great opportunity to quarantine and observe new fish before introducing them to your pond. You’ll have time to treat for any health issues and bond with them in a whole new way.
Indoor Housing Options
Most of these setups range between 100 and 700 gallons of water. Your system size will help determine the number and size of fish you can safely house. All of them require some form of mechanical and biological filtration, which could mean repurposing an old pond filter or purchasing a new one specific to your system’s needs.
Cycling your indoor tank before introducing fish is crucial. One simple way is to squeeze a sponge filter from an established tank into your new system to seed beneficial bacteria. You’ll also want a dechlorinator to remove chlorine and chloramine during water changes, and a water testing kit or strips to monitor ammonia, nitrite and nitrate levels. Ammonia and nitrite can be deadly, while nitrates above 80 to 100 ppm may not kill koi or goldfish, but should prompt a water change.
Option 1: The Most Cost-Effective – IBC Totes
Intermediate Bulk Containers (IBCs) are one of the cheapest and easiest solutions. These food-grade plastic containers typically hold 275 gallons and can often be found used online through platforms like Craigslist or Facebook Marketplace. A new one will cost more but still beats many alternatives.
When buying used, confirm it was previously used for food-grade storage, and clean it thoroughly. Leave part of the top frame intact for rigidity and to help redirect jumping fish back into the tank. The built-in bottom drain is another major perk, simplifying water changes. A net is optional but recommended.
Option 2: The Largest Size – Stock Tanks
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Plastic stock tanks range from 150 to 750 gallons and offer more surface area than IBC totes, giving your fish room to swim horizontally. Their low-profile, wide shape makes them well suited for indoor pond features. Koi, however, are known jumpers—especially in areas with high water movement—so a net or cover is a must.
Most are black, which can impact how fish colors appear depending on lighting. If you’re going for aesthetics, these can be dressed up with potted plants to give the illusion of an outdoor pond indoors. Avoid galvanized metal stock tanks, which may leach harmful zinc into the water.
Option 3: The Most Expensive – Glass Aquariums
Aquariums offer the best viewing experience, especially from the side. But they’re also the most expensive and most fragile of the options. A new 240-gallon tank can run upwards of $3,000, while a food-grade IBC tote or 300-gallon stock tank could cost as little as $150 to $275.
Aquariums also come with risks: seams can leak, and glass is vulnerable to breakage—especially in garages or busy indoor spaces. While they look beautiful, glass tanks wouldn’t be my first choice unless you already have one available. That said, they are perfect for hobbyists wanting to enjoy their fish from every angle.
How Many Fish Can I Keep?
You may have noticed I’ve intentionally avoided giving specific recommendations on how many fish to keep, or what size filter you need. That’s because every system is different. Your fish load depends on your water volume, temperature, filtration capacity and how often you can perform water changes.
Generally speaking, colder systems (around 40–50°F) can support more fish, since they eat less, produce less waste and require less oxygen. Warmer systems (around 70°F) require more filtration and oversight.
As a general starting point, a 4–6 inch fish can be comfortably housed in about 30–40 gallons of water. Experienced aquarists may safely keep more, but remember—more fish means more maintenance. I’ve seen 5,000 juvenile koi (3–4 inches) successfully overwintered in a 400-gallon tank, but that was with a highly specialized setup. That’s a discussion for another time.
There are other creative indoor housing options, too. Used fiberglass tanks are durable and often spacious, though expensive when purchased new. Another quirky option is a swimming pool—if you can convince your significant other! Some manufacturers even offer small pools specifically designed for fish, complete with clear front panels for top-down and side viewing. Whatever route you choose, the key is creating a safe, stable and comfortable environment where your fish can thrive until they return to their outdoor home in the spring.
About the Author: Andrew Lohman is a biologist at Mt. Parnell Fisheries Co., located in Mercersburg, PA. He enjoys managing the fishery’s recirculating system, spawning koi and goldfish each year, and packing and shipping fish for customers, knowing that they will be enjoyed in ponds and aquariums by people all around the country.
