One thing that one learns about pet stores is that they are not selective about what fish they sell.  The background calculations in pet stores are not based on which fish make good pets, necessarily, but on what it’s easy for them to stock.  Some of that is based on popular demand, which influences what fish wholesalers in Florida and East Asia send to vendors around the world, but a lot of it is based on what’s cheap to stock.  And it’s dirt cheap to stock the tiny juveniles of fish from commercial, for-eating fish farms.

A lot of fish sold in pet stores grow WAY bigger than people realize, especially people who grew up on the patently false adage that fish only grow as big as their containers permit.  We in the fishkeeping hobby know these beasts by the evocative name “Tankbusters.”

(Credit for red-tailed catfish image.)

All of these beasts are food fish in their native lands, with the possible exception of the Oscar.  (Koi are a breed of common carp.)  All of them experience stunted growth, bone damage, and all sorts of other health problems when forced to grow up in environments that can’t accommodate their adult bulk, in addition to producing much more waste than a small tank’s filter can typically withstand and eating smaller tankmates.  (The herbivorous pacu will content itself with crushing any fingers or testicles that find their way into its entertainingly human-like, nut-cracking mouth.)  And that’s if it’s even possible to accommodate them.  Very, VERY few people with the wherewithal to comfortably house a shoal of iridescent sharks don’t either charge admission or sell them for food.

Goldfish are also extremely long-lived cold-water fish that don’t do well in tropical fish tanks.  Seriously, everything you think you know about goldfish is wrong.

Fish do not conform to their containers, any more than people in small houses stay small.  It’s vitally important to know how big a fish can get, and to stay far, far away from fish that grow bigger than one can comfortably house.  And that means turning down a lot of what pet stores try to sell, else your perky little catfish will become one more deformed hand-me-down on display at the local public aquarium.

A similar problem is the origin of the fish on sale at your local store.  Water is a surprisingly diverse medium for habitation, and while most of us quickly internalize that marine fish have no place in freshwater or vice versa, there are many grades in between that provide similar challenges.  A small but important percentage of the fish for sale in the “freshwater” section of a pet store are actually brackish-dwellers, adapted to the intermediate salt contents of estuaries and other places where rivers meet oceans.  These include the well-known green puffer, the less frequently seen archerfish and four-eyed fish, and the extremely common molly.  Most of these animals are adapted to variable salt contents, since estuaries usually have a salinity gradient as one approaches the ocean, but that doesn’t mean that these animals enjoy fresh water.  On the contrary—brackish-water fish kept in freshwater typically have abbreviated lifespans because of the extra work required to acclimate and stay acclimated to less salty water in the long term.  Puffers can last years if kept in ideal conditions, but rarely last months in fresh water, in addition to being just a little too nippy for most people’s tanks.  Other brackish fish handle fresh water even less adroitly.  Only the sailfin molly seems euryhaline enough to acclimate to marine, freshwater, and brackish conditions alike, to the point that it’s often used to help cycle new marine systems.

Another ecological feature of most estuaries is a little more worrisome from our perspective.  As a statistical effect of there simply being more marine than freshwater fish, there are a LOT more marine than freshwater fish that visit estuaries temporarily.  Many marine species breed, hunt, or otherwise appear in brackish areas while remaining marine, and these occasionally find their way into poorly-informed pet stores’ tanks, where they are usually housed as “freshwater” animals along with the actual brackish species.  Most of these don’t last more than a few days, if they even make it to the displays.  But a few, such as moray eels, can persist in this state for weeks, long enough to be sold to an unsuspecting hobbyist who thinks they’ve scored the find of their lives.  This is, thankfully, a phenomenon I know only through the letters sent to aquarium hobby magazines, and not something I’ve witnessed personally, unlike the tankbusters above.

All of this becomes a specific example of the next, broader point: know what the hell you are doing!

The various pointers above are all subsets of a single piece of advice: provide your pets with what they need.  That means setting up your system based on the needs of what you want to put in it, and that means picking your fish based on what your system provides.  In this sense, keeping fish is only a little different from keeping any other kind of pet.  Just as different breeds of dogs (and different individual dogs, for that matter) have different needs for exercise, attention, food, brushing, etc, so do different fish species have different needs that a conscientious pet owner must strive to meet.  Of course, pet fish represent hundreds of different species, rather than variations of a single domestic animal, so their needs can be much more diverse.

Take this interesting beast.  The African freshwater butterflyfish is close kin to the South American arowana, and is easily one of my favorite pet fish.  It’s also an animal that I’ve never had the chance to keep properly, because its needs are so specific.  Unlike many better-known aquarium fish, Pantodon is strictly wild-caught in West Africa, so it rarely comes to hobbyists used to or interested in prepared fish food.  It’s a predator that eats fallen insects and smaller fish that it comes across at the water’s surface, which it doesn’t leave unless it’s scared or starving.  It’s also an extremely jittery fish, especially in a tank where it can’t stay near the surface and hide under some sort of cover, such as floating plants.  What usually does Pantodon in, when it doesn’t starve from lack of food it recognizes, is its skill at jumping, which routinely propels the unfortunate creatures several feet away from their tanks if something startles them.  They can survive the fall, but not hours out of water or any cats that might find them.  So a well-fitting lid is essential, which all but rules out the hang-on-the-back filters that dominate the trade.

Or perhaps you’ve spied the evocatively-named black ghost knifefish at your local fish store.  Their swimming method is a wonder to behold, involving the ability to swim forward, backward, and even upside-down with equal ease.  The effect is even more impressive with their bold black-and-white colors.  It’s also a bit of a tankbuster, growing rapidly into a 30-cm-plus behemoth that needs at least a 55-gallon tank just to turn around comfortably, and ideally even more.  It’s mostly carnivorous, eating smaller fish and sinking pellets whole with a mouth that is MUCH larger than its dopey facial features let on.  Its weirdest feature, though, has to be its electrical organ, which it uses to emit a weak electrical pulse that lets it navigate despite being effectively blind.  Fishkeepers have exploited this aspect of the black ghost to create transparent knifefish shelters that let this animal feel secure in a hiding place without being constantly invisible.  These electric pulses also mean that it’s a bad idea to keep more than one fish with a similar organ, including other knifefish, elephant-nose fish, and so on, in the same tank, because they tend to interpret other fish’s electric signals as hostile intent.  They’re a close relative of the electric eel, after all.

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The take-home message here is, don’t do impulse pets.  Just don’t.  Have an idea of what you want before you buy anything, know what species you can and can’t sustain with your setup, and have a very clear idea of what your pets need from you.  You wouldn’t get a big dog if you didn’t have the space for it to exercise; don’t get a hillstream loach unless you can set up a tank that meets its needs, either.

Each fish you meet is a little piece of a far-off (or nearby) locale, an exotic and illuminating experience of the diversity our world has to offer.  If you go into the hobby with all eyes open and with the understanding that you’re not buying furniture, but animals that need your care, you’ll have an illuminating, educational, and beautiful fishkeeping experience.