Can Terrapins Coexist with Koi?

Once you have a koi, you might as well put a terrapin into the pond to keep it company. Or, at least, that’s what many people think.

They have the tank or pond, the filter, and the experience to care for their koi fish and are ready to introduce a new friend to the habitat.

Terrapins and koi, however, are quite different animals and require very specific living conditions. While koi are relatively easy-going, terrapins have more demanding needs. They require water but also a basking area for them to take in the sun.

If that can’t be provided, they need UV lights to make vitamin D which helps them keep a strong shell.

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Terrapins are omnivorous: they will happily eat fish and vegetables, so having koi swimming around them may awaken their hunting instincts.

While keeping both terrapins and koi is feasible, it should be done with good preparation and extra precautions.

Terrapins and koi in a pond

Most koi owners have them in ponds, in natural surroundings, like a garden, a terrace or a yard.

The current law regarding terrapins, however, forbids terrapin owners from keeping their terrapins in unrestricted areas—and a pond certainly qualifies as one.

According to the law that categorised terrapins as an invasive non-native animal, pet owners who have had their terrapin before the 2016 ban may keep their pet as long as it’s not in a garden and is always kept in a contained holding.

Terrapins are curious animals that like to wander, particularly when it’s mating season.

They live in aquatic and terrestrial environments and love to bask in the sun to heat themselves. So, they will inevitably try to climb out of the pond to find a good place to take in the sun. There is no telling if the terrapin doesn’t choose to go further away from the pond—into the garden and beyond.

A terrapin in a pond could happily live there but may eventually try to escape into the wild. With an estimated 4,000 terrapins living in the wild in the UK, there is growing concern that they may start breeding and unbalance sensitive ecosystems, hence the reason for the terrapin ban.

Therefore, you may not typically introduce a terrapin into your koi pond. If you want to have a terrapin and a koi, it will have to be in a tank inside your home, where the terrapin can’t easily escape.

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Terrapins and koi in a tank

Terrapins and koi in a tank need some preparation before introducing them to each other.

Choose the right size of fish

Terrapins will see small-sized fish as prey. If you want to keep terrapins and koi together, you must introduce large-sized koi. If the fish are big enough, the terrapins will think twice before taking a bite at them.

Remember that terrapins have very strong snapping jaws, so you really want a fish that can outswim a terrapin and is big enough not to fit in the terrapin’s mouth. Adult koi will do better.

Choose the right time for your terrapin

Adult terrapins have already reached their maximum size. By now you know what size of koi or other fish you want to introduce to your terrapin tank.

Also, adult terrapins like to have more vegetables in their diet, compared to fish and other live creatures. The fish you will introduce will stand a better chance of thriving if your terrapin is already an adult.

Create a zone for the fish to escape and hide

Your koi will want to have a space to hide if they are chased by the terrapin. Include hiding spots such as upside-down pots and other architectural features where the koi can squeeze in and be safe.

Increase the size of your tank

You want to give space to your terrapin and koi to swim around without bouncing off each other. You need a large tank that provides enough space for both animals to live happily, giving each other the necessary social distancing they need.

When the space is too tight, animals become stressed and aggressive. If you care about your animals’ welfare, you should invest in the right-sized tank to make them all happy and relaxed.

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Reinforce the filtering system

Terrapins can be quite messy. They defecate a lot, so you should already have installed a filtering system in your tank.

Koi like clean, pristine environments, so you need to run the filter frequently to provide the perfect conditions for koi survival. Also, terrapins often leave leftovers after eating. You want to be thoroughly clean and maintain a nice tank for the koi.

Keep the water warm

Terrapins like warm water. The best water temperature for both terrapins and koi hovers between 23 and 26 degrees Celsius and the water pH should be between 7.4 and 7.6. Under these conditions, your terrapin and koi should be happy. You may need a water heater and a water conditioner to ensure the right conditions in your tank.

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Observe your pets

When you introduce a koi to your terrapin tank, you should keep an eye on how both animals behave. It’s best if you only introduce one koi at a time. Too many fish swimming around the terrapin may stress it and it could turn more aggressive.

Before you put the koi in the tank, feed your terrapin well. It will have less inclination to nibble at the koi. Once the koi is in the tank, observe how both animals behave. If you notice aggression from either animal, take out the koi and put it in a different tank.

Terrapins and koi

There is nothing that forbids a terrapin from living with a koi in a tank but you should make sure both animals’ welfare is taken into account. While terrapins can be quite territorial, koi are not aggressive.

With the proper steps and a big enough tank, you could have them living together.

Remember that you shouldn’t leave your terrapin in a pond, even if it feels like you are offering it more freedom of movement. Terrapins can be invasive and we are still unsure how they might challenge the existing ecosystems when in the wild.