Semi-aggressive fish: a handy rule to follow!

Semi-aggressive fish: a handy rule to follow!

  • Author Ouse
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Say you are looking into keeping aggressive, predatory fish after keeping only peaceful community fish during your time in the hobby. You want to progress upwards from where you currently are, but going from keeping peaceful fish to aggressive fish is too big of a leap for your preference. Where is the line drawn between peaceful and aggressive exactly? How will you begin keeping aggressive fish provided you want to progress consistently? Well, let me introduce you to the world of semi-aggressive fish!

What I mean by a “semi-aggressive” fish is a fish that is aggressive indefinitely, so to speak. They are usually peaceful but there are circumstances that can trigger the fish to turn violent and disruptive, such as mating rituals, small tank size or territorial disputes. There is a clear difference between a peaceful fish and a semi-aggressive one, but the difference between semi-aggressive and outright aggressive is less clear. The difference here is that semi-aggressive fish are in most cases calm, paced fish while aggressive fish have the same features of aggression as semi-aggressive fish but upscaled, and are more predatory. Aggressive fish will seldom bully each other to death and are opportunistic feeders of smaller tankmates in the form of fish and invertebrates.

An example of a semi-aggressive fish would be a rainbowfish while an example of a plain aggressive fish would be an oscar cichlid. See the difference now?

This is a good rule to use when deciding on stocking. You want fish of a similar temperament to each other to avoid instances of bullying of dominance that disrupts the equilibrium that is peace. Note that there are some exceptions to this rule, such as plecos, that can thrive in a tank of any overall temperament without having an effect on the aggression level.

Below I will list some semi-aggressive fish that are popular in the hobby. More often than not the temperament of these fish is overlooked and incorrectly estimated by hobbyists.

The angelfish is a popular cichlid fish in the fishkeeping hobby, and it’s already a well known fact that they can exhibit aggressive behaviour. They can be kept with a variety of fish as well as larger cichlids when fully grown: six inches at max! There are a few species of angelfish but pterophyllum scalare is the most commonly kept. Angelfish like to be in small groups and are an example of a fish that prefer tall tanks rather than long tanks.

Another semi-aggressive fish that is commonly kept in the hobby is the rainbowfish. A way the break up tensions between rainbowfish and other tankmates is by providing said rainbowfish with a same-species shoal (preferably at least six individuals) and offering the shoal a large area to swim around. They prefer fast-flowing filter outputs. From the 5cm neon rainbowfish to the 15cm larger species all rainbowfish are semi-aggressive, with the males in particular being the showoffs.

Bettas and gouramis
A group of fish I usually don’t recommend due to their preference in weak currents and susceptibility to dropsy. The gourami family consists of semi-aggressive fish that usually don’t have issues with other tankmates, but will fight and breed with each other insatiably. Fish in this family have a labyrinth organ which allows them to take in most of their oxygen intake from the surface. Bettas are very commonly kept and are very versatile and personable, with aggression and sensitivity to water parameters being their downsides.

Ram cichlids
Cichlids are typically aggressive fish that are unsuitable for the standard community, but rams cichlids are a bit different. Bolivian rams, German blue rams and electric blue rams are all cichlids that can be kept in communities with a variety of typical community fish with success, as long as gender and breeding is considered carefully. Having a mix of male and female rams will likely result in courtship fights breaking out in the tank. Rams are quite small as far as cichlids go, staying under 10cm, with species varying in max size.

So there you have it. I hope this rule helps you choose what stocking is best for your aquarium!
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  • ZachG
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Very helpful guide, crusty dusty is pretty cool!
Your compliments are appreciated!
  • Catappa
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Good information that should help members make more informed decisions about the fish they choose.
Nice work on this article, CrustyDusty! This is a great general guide for anyone who is considering keeping any of the fish listed. You could even include a few more examples of Semi-Aggressive fish such as Red-Tailed Sharks, but great work none-the-less.
Thanks so much eventually I’ll get up to improving the article.
Great information on stocking tanks with semi-aggressive fish and super helpful!
Thanks! :)
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