How to Set Up an Indoor Tank Over Winter

Perhaps you have decided to move your goldfish inside over winter; maybe your pond is too small to prevent freezing, or maybe you are due for harsh snowstorms and are worried about your pets wellbeing.

For whatever reason to bring them inside, you may need to set up a temporary indoor aquarium for your goldfish, until the harsh weather goes away.

But do I need to do this? How? Are there alternatives?

Should I bring my goldfish inside for winter?

Whether or not you decide to take your goldfish indoors is really up to you, however, there are factors which can mean that you absolutely should bring them inside.

One of these is if the pond is too small or if it is poorly insulated, we say that a pond needs to be at least 3ft deep to prevent it from freezing fully – this is plenty of space for the fish to get away from the cold.

Even if the surface freezes over, the fish still have plenty of safe space underneath the ice where they can hibernate in peace.

Another factor to this is insulation. Raised ponds need plenty of thick insulation around them to avoid ice forming from the sides.

Slim raised ponds with thin walls will form ice and will encase the goldfish from above and the sides.

Can goldfish survive in a pond in winter?

If your pond is well insulated and is 3ft deep, then you needn’t worry about leaving your goldfish. They are perfectly capable of living through the very coldest months on their own.

During winter, goldfish enter a dormant hibernation state called torpor, where they conserve energy and need to be left alone until the warm weather returns.

As far as care in winter goes, all you need to do is make sure there is a gap in the ice so that oxygen can still enter the pond. Maintain the filters and check the fish are healthy and okay.

We also have a more in depth guide on pond winter care.

Can pond fish and koi survive indoors?

Koi and goldfish can certainly adjust to life indoors, although they do need quite a heavy setup, especially if you plan to keep them inside long term.

Carp are very large fish that produce a lot of waste. They are like this because their lifestyle consists of consuming large amounts of low nutrient foods like algae, plants and infusoria all day long.

Imagine them like an aquatic horse or cattle; they spend all day grazing and that food has to go somewhere, meaning that goldfish poop a lot!

The amount of waste goldfish and koi produce is very high as far as fish go, and so they require a larger aquarium with strong filtration in order to keep the balance of clean water up.

They need more consistent water changes and are a more high maintenance fish than many others.

Be aware that goldfish can reach 1ft long and koi can reach 3ft, so they also need a very large tank, even if it is only for a few months in the year. You couldn’t squeeze a koi carp into a 20 gallon, as it is the same size as the tank!

Red goldfish looking through glass

Are there any other ways of keeping fish indoors?

There are a number of other methods you can use to house fish, with varying levels of costs, effectiveness and safety.

Glass is a very expensive material, and a large aquarium can set you back a small fortune for something of quality.

However, there are cheap tanks out there; fish stores will often flog old tanks they once used for stock for very low prices. Carboot sales, expos and even social media are all good places to find cheap glass tanks!

You don’t have to use a glass aquarium; there are other methods of holding large volumes of water that can temporarily house fish.

One of these is a simple water tote. It is large enough to house many goldfish and even a few koi over winter, is strong enough to hold high volumes of water and is affordable.

Large plastic containers are also usable, however, they must be able to withstand lots of weight without bowing on the sides. Check the packaging and labels to see how many kilograms it can withstand.

Flexible plastic should be avoided – go for strong rigid plastics like ABS or polycarbonate.

You can also use ceramic sinks or baths to house the fish, although these are much more expensive and very difficult to move around, unlike the plastic container.

As long as the container can hold water without breaking or bowing out, is insulated, or at least kept in an insulated room and can house filtration, then it can definitely be used at a makeshift indoor pond for your fish.

What to consider when bringing pond fish indoors

Timing is key when you decide to move pond fish – moving them too late in the year can shock and kill them if you are not careful!

When moving them, you need to be aware of your fish’s current state of activity – this will be based on the temperature. 

If below 10C / 50F, the risk of stressing your fish by moving them to a warmer environment is high. Stressing them too much can cause illness and health deterioration.

Below 5C / 41F, the risk of stress is very high, as by now the fish have entered hibernation, so disturbing them and increasing the temperature can shock and even kill them!

It is always best to act early on your pond. The latest you should ideally be moving fish around is early november, any time after that, interference with the fish causes too much stress.

Bringing fish out of torpor is dangerous as it can shock their body, however, you may be struck with a dilemma where the only time you can move them is late in the year when they are already hibernating.

If this is the case, then we suggest moving them to a cold room, like a garage or shed, so they will not freeze, but will stay cool enough to remain in hibernation.

If this is not possible, then you need to be very careful when bringing them inside. Acclimate them for a long time; perhaps take buckets or containers full of their icy pond water to fill the indoor pond.

Acclimate them slowly, so as to avoid shocking them. It is possible to do, but take things slowly.

You also need to consider the new level of maintenance you will have to do, especially if the tank or container is not cycled. You need to keep on top of those water changes and have a strong filter to prevent toxic ammonia.

Black beard algae in Tank wit rocks and plants

How to set up an indoor tank for outdoor pond fish

Setting up a temporary indoor tank for your pond fish is fairly simple. As mentioned, you need a large enough aquarium or container to hold the water, with adequate filtration.

We would recommend using pond water to fill the tank as it will be the same temperature the fish are used to, and the hours of acclimation will not be needed, therefore minimising stress.

Make sure the filter is running properly and we would suggest using a filter that does more litres than your tank holds, to help better control the high bioload of goldfish.

You could also add air stones for additional aeration, which we always recommend for aquariums as they aren’t as good at absorbing clean oxygen as a pond is.

Once set up with a filter and filled with water, you may move the pond fish, but you may be wondering about cycling.

  • Don’t worry, you can use a bottle of live bacteria colony to help kickstart the cycle. 

We would recommend dosing a quarter or half the bottle when you first add the fish, then consistently add small doses over the next few days.

Some bottled live bacteria is great for when you need to set up a fish tank right away. After 1 week, we recommend doing consistent water changes however, keep up with 20% or more every week.

How to return your fish to the pond after being indoors

Returning fish to the pond is quite simple, but you need to wait until it gets warmer and the weather outside becomes more consistent and stable.

Usually by late March to early April is when things tend to settle down, although nights can still reach freezing temperatures up until late April some years.

In short, you want to wait until the temperatures get above 10C / 50F during the day at least; that way, the fish do not need to adjust to a different lifestyle, and they can still be active and eat the same food.

Having the pond temperature closer to room temp minimises stress on the fish.

Make sure to use an accurate thermometer to better decipher when the best time is to add your fish to the pond.

What to feed indoor koi and goldfish

During the autumn and early winter months, we move our pond fish onto wheatgerm.

During the dead of winter and early spring, we don’t feed them at all. So if you move them indoors during winter, what do they eat?

Fish work with the temperature of the water around them, and the only reason they don’t eat during winter is because it is too cold for the bacteria in their stomach to digest food.

Once they warm up in your house, they will go back to normal and can be fed on their normal foods again, but introduce them back slowly. Start by feeding them wheatgerm only for a few days, until they fully warm up.

After they have settled in for about a week, treat them as normal aquarium fish, but be sure to keep on top of those water changes, as they are extra messy.

Leave a Comment