Aquatics Unlimited: Articles: Victims - and Criminals - of Circumstance
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"Victims - and Criminals - of Circumstance "

If you felt I was a little off base in the previous installments - in which I implied that some fish were such tempting targets that they practically begged to be nibbled, chewed or gulped - well, brace yourself for this one. In many cases, the responsible party for aquatic assaults isn't the aggressor, or even the aggressee. Instead, the finger of blame points squarely at the one who is ultimately responsible for all the goings-on in his little slice-of-nature-in-a-glass-box: the aquarist.

Obviously, we fishkeepers bear a certain amount of accountability whenever fish take a dislike to one another in our aquariums. After all, if we didn't put fish in aquariums where there is a risk of being harassed, maimed or murdered, they'd still be swimming in some lake or river somewhere - where there is a risk of being harassed, maimed or murdered. But oftentimes our error isn't simply a matter of picking incompatible tankmates, but is instead a matter of making tankmates incompatible by subjecting them to unnatural circumstances.

A number of aquarium "parameters" (from the Latin para ("stuff") and meter ("you can measure"), have an effect on fish compatibility. Some (like the number of decorations in a cichlid tank) have been covered in previous installments, but here are a few more that I see regularly:

Break-in bullies. As a new (or thoroughly cleaned) tank goes through that first 4-6 week "break-in cycle," the high levels of ammonia and nitrite seem to make some fish downright irritable. Fish like tiger barbs, red minor tetras and even black tetras, which are usually nippy at worst, seemingly take a great dislike to other specimens and pursue them mercilessly until they are separated (either by the aquarist or Old Uncle Death) - or until the day the nitrite cycle ends. The key to breaking in a new tank is to select all hardy startup fish of about the same size and aggressiveness.

Old-tank syndrome. We can generally add a new fish to even an aggressive community - provided he is somewhat bigger, tougher or smarter than the existing population. However, sometimes that new specimen becomes instantly listless and just sort of hangs there in the water while the "older" fish skin and scale him. More often than not, very high nitrates - caused by insufficient water changes in the older aquarium - are the culprit. The original fish have had a chance to slowly become accustomed to the nitrate levels as they rose over the past months, but the new specimen pretty much goes into shock upon being placed in water with nitrates a hundred or two ppm higher than that to which he is accustomed. This calamity can occur in any aggressive or semi-aggressive community, but is most commonly seen in tanks of "lunkers," such as oscars, tinfoil barbs, silver dollars, pacus and the like. The same fish might well have survived if the tank had been properly cared for.

Bigger is better. Larger aquariums have a number of advantages over smaller ones. For example, they are harder to overcrowd and more chemically and thermally stable. In addition, they provide a little elbow room, so to speak, for fish interactions. Many otherwise impossible fish combinations, such as tiger barbs and angelfish, have occasionally been successfully maintained in aquariums of 70 gallons and larger.

Now we're cookin'. Some fish, particularly African cichlids of the Psuedotropheus and Melanochromis genera, seem to become homicidal maniacs if the temperature goes beyond a certain point. That's bad enough, but other fish that are often kept with them (for example some of the Labidochromis and Lamprologus species) have exactly the opposite reaction to higher temperction to higher temperatures, becoming sluggish, weak and disoriented. At the typical 75-80 degrees, everybody gets along fine, but above 85, it's Freddy Krueger vs. Beetle Baily.

  • tiger barbs = Barbus tetrazona
  • red minor tetras = Hyphessobrycon callistus
  • black tetras = Gymnocorymbus ternetzi
  • oscars = Astronotus ocellatus
  • tinfoil barbs = Barbus schwanenfeldi
  • silver dollars = Metynnis and Mylossoma sp.
  • pacus = Colossoma brachypomum
  • angelfish = Pterophyllum scalare

This article originally appeared in
Freshwater and Marine Aquarium Magazine

Copyright © 1997 James M. Kostich
All rights reserved.

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