Originally published in the Marine Aquarium Society of Colorado Newsletter, September 2009
I'd like to say up front that I'm as fond of the standard saltwater aquarium-keeping books as the next guy. Bob Fenner, Julian Sprung, Anthony Calfo, they are all good places to start when you are just getting going in the hobby. There is one recurring theme in many of the guides for beginners that kind of drives me nuts, though. I'm paraphrasing here, but I bet you've heard it, too:
I'll grant you, limiting yourself to one fish of any one species can help you to avoid some headaches. Some species, especially if you wind up with two males, can get really nasty to each other. But with most species, it's easy to avoid the problems and the conspecific interactions can add a whole new world of interest for you in that little glass box you've created. And, if you aren't careful, explorations in this direction can take you on entirely new journeys into esoteric realms of the hobby--for example, trying to raise the larvae that might be the fruits of your efforts--that can certainly help convince your spouse and/or friends that you've truly gone off the deep end at last. But even if you have no intention of trying to raise any larvae--yet--keeping pairs and groups of fish can reward you with all manner of interesting behaviors that can entertain you for years after the thrill of getting your aquarium running for the first time has worn off.
Actually, it seems to be getting easier to find dealers that sell pairs of fish. Part of this, I think, is that the hobby seems to be moving toward keeping multiples of species. But I think the largest motivation is that dealers have found that they can charge their clients much more money for a "pair" of fish than if they were selling each fish singly. I say "pair" with quotation marks because, for the most part, dealers are not selling fish that are paired in any true sense. "Pair" usually seems to mean, "In the two weeks these have been in our tanks they haven't killed each other." So, what you are really paying for is two fish that have the potential to become a pair. And, heck, you can do that, right?
Unfortunately, pairing fish takes some homework. There are some tricks involved and they are all about knowing the species of fish that you are trying to work with. So the first thing to talk about is the different ways that fish make pairs in the wild.
Now that most of us are in the internet age, it's become painfully obvious that human sexuality is ... well ... complicated. Oddly enough, we don't hold a stick to some of the things they get up to under the water. But if you want to try to pair fish, you have to know what the fish you are interested in look for in a potential partner. Here are the big strategies that cover most of the fish you might meet:
As you can see, the sexual strategies of the fish you want to keep makes a huge difference in how you try to pair them. Let's start from the easiest species first.
Nothing could be easier, right? You just buy any two fish and put them together and they should form a pair eventually. Feed them plenty and you will get to watch the amazing social rituals as the fish sort themselves out. Unfortunately, there aren't very many species of fish that practice this sexual method. One of the main groups that practice this strategy are the "coral croucher" gobies. I have a pair of Citron gobies and the social games were very interesting. They often exhibited "king of the hill" behaviors--each trying to get higher than the other--all while wildly shaking their oversized fins at each other.
Theoretically, pairs of these should be almost as easy to form as pairs of the bidirectional hermaphrodites and many, many fish species fall under this heading. Obtain any two juvenile fish of the same species, place them together, and they should eventually form a pair. In practice, though, it's a good idea to do some research on your species to see if there are any problems making pairs.
For example, clownfish (depending on the species) are nearly foolproof with this method. But the maroon clowns will often kill their partner as part of trying to force their partner's sex change and should be kept in a partitioned tank where they can see and smell each other but cannot physically interact until the pair bond has formed.
Similarly, many species of dottybacks are extremely hard on their partners during pair formation and sometimes even beyond. Keeping the smaller individual in a mesh cage inside the male's tank is the best practice. Once the two seem paired, cut a hole big enough to allow the smaller individual access in and out of the mesh cage and too small for the larger individual to enter. That way, the smaller individual always has a safe hiding place if the other gets too rough.
But, what do you do if there are no available juveniles of the species you want to keep? Or, if you already have one large individual of the species and want to pair it? Often, you can still pair them, if you find as small an individual as possible to match to as large an individual as possible. You have to keep a close eye on them to make sure that the aggression doesn't get too great. Sometimes the smaller individual's sex will already be fixed and there may be nothing you can do.
Forming pairs of fixed-sex fish can either be extremely easy or extremely difficult. If the two sexes of the fish species look differently from each other then the pairing is really straight-forward. Find one individual of each sex and see what happens. For this to work, though, you have to do some research and find out what the different sexes look like. Fortunately, there are lots of sites online where people have posted photos of the different sexes of many species of fish.
On the other hand, there are many species where the differences between male and female are either non-existant (most tangs, pearly jawfish) or are only evident just before or just after mating (blue spot jawfish change colors for a courtship display, Banggai cardinalfish males can be told easily once they are holding eggs). With those, it really is a matter of trial-and-error and watching the reactions of the fish that will guide you to making a pair. I keep one known Banggai male separate from other Banggais so I can use his reactions as a way to tell the sex of any new fish. He knows the difference, even if I can't see it.
I've found that organizing social groups of fish has added greatly to the challenge and to my enjoyment of aquarium keeping. For many species of fish, establishing these social groups is not an overwhelming challenge and can open up a new vista for your hobby. If you have any questions about this article or pairing fish in general, please feel free to give me a nudge.
Wittenrich, Matthew. The Complete Illustrated Breeder's Guide to Marine Aquarium Fishes. TFH Publications (June 15, 2007).