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How to Introduce a New Fish to an Older Tankmate for Pairing
One of the most magical things you can witness in your reef aquarium is a bonded pair of fish. Whether it's a pair of mandarins gliding side by side or a duo of clownfish dancing in their favorite coral, paired fish are fascinating to observe and often display more natural behaviors. But how do you successfully introduce a new fish to an older tankmate in hopes of creating a pair? With patience, a plan, and the right tools, it can be easier than you think.
Why Pair Fish?
Some fish naturally live in pairs or harems in the wild. When kept alone, they may do just fine, but when paired, they often become more active, social, and even display breeding behavior.
Step 1: Choose the Right Match
Clownfish:
It may seem counterintuitive, but if you already have a Clownfish, you must introduce a smaller, younger Clownfish. Clownfish are protandrous hermaphrodites, meaning the dominant fish will become female, and the smaller will become male. Introducing two female Clownfish together will almost always end in tragedy. That's why it's important to choose the new Clownfish from a batch of clownfish that are most likely still juveniles. If you put two Clownfish together that have been single for a while, there’s a risk that both have already transitioned to female.
Mandarin Fish:
Mandarin Fish must be kept in male/female pairs. Though they are born either male or female and do not transition, it can be difficult to accurately sex them, so feel free to reach out to us with a photo of your Mandarin with the dorsal fin fully erect.
Our customers have successfully paired Mandarins of all sizes and ages using an acclimation box. On the rare occasions that pairs simply will not get along, a common element in these situations is a too-small tank size. It's imperative to only try pairing Mandarins in an aquarium of at least 30 gallons.
Angelfish:
Angelfish can be difficult to pair when the individuals aren't already familiar with one another. That's why purchasing two captive bred angelfish at the same from the same batch is ideal. This is much easier than trying to pair two wild-caught angelfish who are not familiar with each other, and their age/gender/maturity is unknown.
Angelfish are protogynous hermaphordites, meaning they start life as females, and the dominant individuals mature into males. If you already have an older Angelfish, and you want to introduce another, you must choose an Angelfish that has not yet transitioned to a male. 
Step 2: Use an Acclimation Box
This is the most important tool in your pairing setup. An acclimation box is a clear, ventilated container that hangs inside your aquarium, allowing the new fish to be visible to the older resident without risk of aggression. Both fish can see, smell, and interact with each other through the box for several days.
We recommend keeping the new fish in the acclimation box for ~7 days, depending on how the older fish reacts. Watch closely for signs of interest vs. hostility. You want curious behavior like hovering near the box or dancing movements, not flaring, nipping, or charging.
Make sure the acclimation box is large enough to comfortably house the fish and has hiding places inside like small PVC pipes or rocks. Also consider the size of your fish and ensure the ventilation holes are not large enough for your fish to escape. Biota's custom PNW boxes are designed for some of our smaller fish like Mandarins.
Step 3: Watch Their Body Language
Mandarins are especially expressive. If you’re pairing a male and female, you may notice them hunting together along the sides of the box or displaying gentle body movements. Clownfish may hover near the box or try to "host" near it. These are all good signs.
Step 4: Release at the Right Time
Once the fish are comfortable and showing peaceful interest in each other, release the new fish into the tank during a low-stress time, ideally at dusk with the lights dimmed or off. This helps reduce aggression and gives the new fish time to settle in before a new day.
Step 5: Keep Observing
After the release, continue to monitor the fish closely. Some chasing or posturing is normal as they establish their roles, but anything too aggressive or relentless should be taken seriously. In that case, it’s okay to separate them with the acclimation box and try again in a few days.
If you're noticing a lot of aggression on the first introduction, it is ideal to sequester the aggressor and allow the new fish to explore and gain territory if possible. With highly territorial species like some clownfish, you may need to repeat this process several times.
Mandarin Success Stories
We've heard from countless Biota customers who successfully paired our captive bred mandarin fish with their existing mandarin fish over the years. Using the acclimation box method, many hobbyists have created peaceful, bonded pairs, even in tanks with established, older fish.
When paired successfully, mandarins often travel the tank together, hunting copepods and exploring their territory. In the evenings, they perform an elegant courtship dance, rising slowly together into the water column.
A Few Extra Tips:
Make sure your tank is large enough, has plenty of hiding spaces, and has a lot of rockwork
Feed both fish well before and after pairing to reduce food-scarcity stress
With a little effort and observation, pairing fish can be a beautiful and rewarding part of the reefkeeping experience.
Have questions about pairing fish or choosing the right match? Reach out to us, we're happy to help you find your fish's perfect partner.
Are you considering adding a group of tangs or another territorial shoaling species? We have a blog just for you: https://shop.thebiotagroup.com/blogs/news/how-to-mix-multiple-tangs-in-an-aquarium
Mandarin photos courtesy of Christina Barringer of finatic_reef on Instagram
