Guidelines for a mixed species tank. Why do some succeed and other's don't?

Guidelines for a mixed species tank. Why do some succeed and other's don't?
by Frank Magallanes (April 25, 2005)

1. Do not make assumptions. Piranhas are considered both scientifically and realistically unpredictable when it comes to bite suppression. Keeping groups of various species together without major damage from bites can vary from a few seconds to a couple of years.

How does a piranha or pirambeba survive within a community of piranha? What about non-predator fish? In order for prey to survive and reproduce, they have evolved cryptic coloration (mimicry) to escape predation (Hunting and killing another animal for food). The colors that we as humans see are not always the same colors that fish see. That may help explain why some dark, bland goldfish are not eaten while the more golden, brighter goldfish are. Some goldfish may have markings that inhibit the bite suppression in piranhas. Or more likely they are not releasing fright response pheromone.

Camouflage or the ability to be unseen by a predator by blending in with its surroundings also helps to increase bite suppression. Freezing on the spot, flicking fins or fleeing are all responses to avoid being bitten by a predator. Piranhas and pirambeba also practice this response. Hobbyist in general think their fish are not afraid. But in reality, piranha and pirambeba are very afraid and display this by being immobile or fleeing. Add sufficient roots and rocks to form caves. This will allow the fish a resting-place and to hide if things become to aggressive. Piranhas use group techniques (or wolf pack method) in order to separate a fish from a large shoal or school. This makes it easier for the piranha to eat it if the prey can be separated from the main group.

2. Aquariums should be the largest available that you can afford. Piranhas and pirambeba should have sufficient space to develop a comfort zone; to include territorial spaces (approximately 2 feet square from each other). The closer a species is to each other the more likely it will increase stress and the bite response will surface. Hiding areas should consist of sufficient live plants (piranhas eat plants as part of the minimal diet). Plastic plants should be avoided as piranhas and pirambeba will bite them and that is not healthy for them and could cause filter problems from small pieces not digested.

3. Conditioned period. Allowing a group to become adjusted to each other is where most of the problems originally begin. Until they are comfortable in their surroundings (which may take up to a few days to a few weeks), expect biting to fins and other aggressive behavior from the more dominant species. From a broader perspective, its best to start with approximately same sized species, introduced together with no lights turned on. Majority of moralities occurs when lights are on or within a few days of introduction. New introductions are likely to be bitten or eaten as they attempt to adjust to new surroundings. Conditioned feedings will also encourage population to eat new arrivals as food.

4. Water changes, often overlooked by aquarists should be performed as often as necessary (usually 3x's a week or more). A water change removes organic and inorganic matter, reduces parasites and promotes growth. Also it will stimulate fishes to breed. Water temperature should not be more than 2-3 degrees different from the aquarium temperature. Anything lower or higher can lead to stress or death, particularly to weakened fish. Expect some heightened aggressiveness or fright response after each water change. It's my suggestion you do the water changes with as much minimal outside disturbance as possible to include allowing the water to simply trickle in as opposed to allowing the water to rush in.

5. Behavior of piranha and pirambeba:

Expect periods of bite suppression from either species.

a) Territorial disputes (ie: one species entering another's domain) can lead to biting or aggressive behavior to include chasing. Majorities of fin bites from pirambeba are limited to fins and that regenerate within a few days.

b) Be watchful for any piranha or pirambeba that has received flesh bites. These can lead the more powerful species to attack and eat a species if blood and/or pheromone (fright response) are being released.

c) Pirambeba employ an open water stalking approach. They do this from the rear and get as close as possible to the intended target. Usually, within a few inches. This minimizes its visibility in order to strike at the fin. If the prey is small enough the pirambeba will give chase to eat it. Often it is satisfied just to eat the fin. It’s been widely reported and seen by this author that pirambeba will eat only the back portion of the flesh (caudal peduncle) and tail. The remaining portion of the fish is left to float and rot. Both piranhas and pirambeba can be observed to approach a target fish in a successful attempt to bite it. Then suddenly turn away as if disinterested. According to research (Brush, 1981), it was revealed an average of approximately six approaches per trial. It was variable, even though the approach seemed genuine, but was aborted. Its possible according to this research, the fish was simply investigating the target fish. Curio (1976) and Brush (1981) believes that piranhas will often try to appear harmless or uninterested in the presence of their potential victims. This goes hand in hand with why they approach a target fish so often. It creates a false sense of security between pirambeba and piranha. The same can be said of any prey fish included in the aquarium. This inattention (or learned behavior) can lead to damage or mortality on the target fish. Fish have low attention spans and this observation does not go unnoticed by potential predators.

d) Yawning: hobbyist who observes piranhas and pirambeba occasionally sees this observation. It is often asked about and wondered why their fish do that. Magnuson and Prescott (1966) proposed for the Pacific Bonito, that yawning has both olfactory and stretching purposes. Pirambeba have large mouths when compared to other piranhas and it is plausible explanation of why the stretching exercises take place. According to experts, the olfactory interpretation is more complex. The reason being, conditions in aquariums are usually “high visibility” meaning there is a reduced need for olfactory.

e) Biting prey in half: Foxx (1972) found an interesting phenomenon when piranhas or pirambeba bit goldfish in half. The head of the goldfish would stay alive for approximately 30 minutes. When Foxx surgically severed the heads of goldfish to find out how long it would live, the approximate time was three to five minutes. Foxx asserted that the teeth of the piranha automatically close the wound after severing the flesh. An uncertain question remained why this would be advantageous. It is my opinion that the reason why this happens, allows a renewable resource should the fish not have a severed head. It allows the fish to heal and be available for another strike.

6. Temperature: Piranhas are warm water fish that do well within a range of 75F to 88F. Cooler water tends to inhibit the bite suppression and feeding behavior.

7. pH values: Maintain pH within neutral 7.0 (preferable) or between 6.4 to 7.8.

While this guideline for mixed species gives a hobbyist some basics to work with, always remember that piranhas and pirambeba are animals. They should not be looked as mechanical objects that can fit into a specific mold. They learn much on instinct and conditioned behavior. As much as we would like them to behave a certain way at a certain time, they are unpredictable as living organisms. Expect moralities and be ever watchful for changes in behavior. In their world you are an intruder and a potential predator. In our world you are an aquarists with ethics with professional and moral conduct to follow while you care for a living organism entrusted to you.

REFERENCES:

Foxx, RM: 1972 Attack Preferences of the Red-Bellied Piranha. Animal Behavior 20:280-283.
Nico, L G & D C Taphorn: 1986. Those Bitin Fish from South America. Tropical Fish Hobbyist February: 24-41, 56-57.
Magnuson, J.J. and J.H. Prescott, 1966. Courtship, locomotion, feeding, and miscellaneous behavior of Pacific bonito (Sarda chiliensis). Anim. Behav. 14:54-67.
Brush, George S. 1981. Predation, Aggression, and Courtship in the White Piranha (Serrasalmus rhombeus), unpublished thesis, Department of Biology, Princeton University.
Curio, E. 1976: The Ethology of Predation. Springer-Verlag, New York.
Langkilde, T., Shine, Richard, & Mason, R.T. Predatory Attacks to the Head vs. Body Modify Behavioral Responses of Garter Snakes. Ethology 110, 937-947 (2004) O 2004 Blackwell Verlag, Berlin ISSN0179-1613.

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UPDATED: 07/09/2009