Aquatics Unlimited: Articles: If the guppy fits - EAT IT!
Image 1
Image 2
Image 3


"If the guppy fits - EAT IT!"

Of all the factors that determine which fish will be compatible in a community aquarium, relative size should be about the most obvious. After all, who would dream of putting a minnow with a bass? - or a mouse with a snake? - or a peanut butter cup with a dieter? The natural relationship between predator and prey is simply not to be denied! Although the behavior of predatory fish cannot really be called aggression (we'll talk more about aggression in a future installment), the result is pretty much the same from the eatee's point of view. It's important, then, to be able to determine which fish are likely to make a meal of one another.

Nearly all fish are predatory if given the opportunity to consume their victims whole. Furthermore, they seem to operate on the principle: "If anything fits into my mouth, it goes into my mouth." Thus we can determine who eats whom not by comparing the size of each fish, but by mentally trying to insert the smaller fish into the larger fish's mouth. (I guess you could try it physically too, but that tends to be a bit more expensive.) Some fairly large fish like blue gouramis and iridescent sharks have relatively small mouths and can swallow only very small tankmates. Other specimens like the pike livebearer and the shovelnose catfish seem to be nearly all mouth and can gulp down fish nearly half their length. Still other species such as spiny eels and arawanas have much larger mouths than would appear possible, and fool us into putting them in with bite-size buddies.

Here are some specific fish to keep an eye on, either as villains or victims:

The ninety-day wonders. Many hobbyists logically assume that any fish that are kept together in the dealer's tanks will get along happily ever after. Everything seems fine for a time, but hey, aren't there a couple of fish missing?! Then, next month a few more and finally, the next month all that's left is a six inch oscar with an enormous smile on his face! Oscars are of course one of the most popular fish that frequently outgrows its tankmates. Other "wonders" include most cichlids, arawanas, clown knife fish, tire track and fire eels, and Columbian cats (see "night gobblers").

ScorpionThe night gobblers. A few fish, in particular certain types of catfish and knife fish, are nocturnalscorpion fish predators. That is, they either hide or patiently cruise the aquarium during daylight hours, but hunt incessantly from the moment the lights go out. Any bite-size tankmates unfortunate enough to be in their paths promptly become midnight snacks. Popular "night gobbling" catfish include the Pictus catfish (sometimes called the "angelicus" catfish) and the Columbian catfish going by various names such as "high-fin bull shark," "silver shark" or "Columbian white tipped shark," both of whom have long, feeler-type whiskers to help locate their prey in the dark. Any fish that comes into contact with the whiskers is promptly seized, and if a suitable size, swallowed whole. All species of knife fish are predatory, and it is strictly a matter of mouth size versus prey size that determines which fish can be kept with them. A clown knife fish can easily inhale a fish up to about one-fifth its length, especially if the fish is a slender-bodied species like a danio or swordtail. Most other knife fish like the black ghost and African knives have smaller mouths, but can still slurp up a tetra or two. Knife fish hunt by sight, and have specially adapted eyes that improve their night vision.

The psychedelic brine shrimp. Alas, one of the all time favorite aquarium fish is just too irresistible a snack for many of our other fishes. I am referring of course to the neon tetra, whose bright colors, streamlined shape (pill manufacturer's should take note), tiny size and almost pitiful acceleration make them the ideal lunchmate for every angelfish, barb and gourami in the neighborhood. And if all that doesn't work, they lie down on the bottom of the tank every night and wait for the "night gobblers" to get them. Their one natural protection, that of travelling in massive schools to confuse predators, is of little value to them in the confines of an aquarium.

The double-whammy. Smaller catfish, especially Corydoras species, are sometimes caught by other bottom feeders like large goldfish and Japanese koi. Unfortunately for the would-be predator, the catfish panics, erects his dorsal and pectoral spines, and locks himself into the soft tissue of the mouth. Unless the aquarist performs some awkward and gruesome "surgery," both fish are usually lost.

Fish discussed in this article:

  • blue gouramis = Trichogaster trichopterus
  • iridescent sharks = Pangasicent sharks = Pangasius sutchi
  • pike livebearer = Belonesox belizanus
  • shovelnose catfish = Sorubim lima
  • spiny eels = Mastocembelus sp.
  • arawanas = Osteoglossum bicirrhosum
  • Oscars = Astronotus ocellatus
  • clown knife fish = Notopterus chitala
  • Pictus catfish = Pimellodella pictus
  • Columbian catfish = Arius jordani
  • black ghost knife = Sternarchus albifrons
  • African knives = Xenomystus nigri
  • neon tetra = Hyphessobrycon innesi

This article originally appeared in
Freshwater and Marine Aquarium Magazine

Copyright © 1997 James M. Kostich
All rights reserved.

Aquatics Unlimited | 3550 S. 108th Street | Greenfield, WI 53228
Monday-Saturday 10:00 a.m. - 9:00 p.m., Sunday 10:00 a.m. - 6:00 p.m. | Phone: 414-543-2552 | Fax: 414-543-4929