That cute little fish you bought could turn into your biggest headache if you don't think before you purchase, so here are a few words to help you in your selection when considering buying that little cutie. Also here we will be talking about which fish will live with others in harmony. although even after careful selection, we sometimes get a small fish that will bully others a great deal bigger than themselves, but luckily this mainly happens at breeding time.
Any fish can become a rogue, it doesn't necessarily have to be a big fish, as even some small species of fish can become quite dominating at times, especially at breeding time.
Generally when we speak of rogues people tend to think of the bigger species like some of the Cichlids,
and many of this group do get to a stage where they will only tolerate fish either the same size, or bigger than themselves. Other types of rogues are the ones that will uproot everything you plant, move vast amounts of gravel that leaves your tank like a moonscape, and even move small rocks if they are in their way. Others will ram the heater against the tank side, or pull the uprights from the undergravel filters, and on occasions decide to knock the lid off the tank and try their hand at jumping from the tank and onto the floor. Many of the big Cichlids will attack your hand if you try to clean the tank or move something that will alter their environment, while others will practically roll over and let you scratch their belly. Just by the last sentence we can see that not all Cichlids are the same, so we can't brand any fish as being a rogue.
So what causes a fish to be aggressive? Several things can cause this, many of which we can do our best to avoid. Fish near or actually spawning, or looking after young should never be trusted with other fish if the are of an appreciable size. At this time adult fish become very possessive about territory and the safety of their young, so now is one time you should leave well alone and let the fish get on with what they were born to do. In some situations we may have to remove either one or both the parents, but Cichlids in general tend to take care of their young right up to the time they are ready to spawn again, and it has been known for them to care for two broods of fry only weeks apart. Many of the Mouthbrooders also do this and can have more than one brood all trying to retreat into the mother's mouth. Great stuff.
Many Cichlids are very selective about their partner, and many once mated, will remain with that same partner to the end. If one dies, then it is sometimes hard to get the remaining fish to accept a new mate, and I have found this to happen with the Angels, the Jewel Cichlids, and the Firemouths, but again, not all fish are the same, but in general Cichlids seem to create a very strong bond with the partner they choose. In more than one instance I have had the remaining fish die within a short while after losing its partner.
Rogue fish are generally territorial fish and will set up an invisible area that they call theirs and will not allow outsiders to move into their area. Not much can be done here, but re-arranging the tank sometimes helps by confusing the fish as to where his territory starts and ends.
If the fish in question is causing harm to the more docile occupants of the tank, then your only option is to either move the aggressor, or move the peaceful fish. One should really consider a spare tank for these situations, and when not in general use it can be used as a quarantine tank, or for holding newborn fry, but it should always be setup correctly at the right temperature and ready for any emergency.
Even the common Zebra Danio can become aggressive if others in the tank are smaller or not quite as quick as it is. Keeping just one of these fish tends to cause this problem, especially if the Zebra has grown to an appreciable size.
Take the little Dwarf Gourami. A wonderful colourful fish that is usually quite docile, but catch him at breeding time and it's a different story. This little fellow can be a real terror if he is guarding the nest and will send most fish twice his size running in fear to the other end of the tank. This will happen with most of the Gourami species, and the Bettas.
So what can you do to stop it? If you have a fish that bullies others you could try to find something to distract it. Many fish that bully are single species in a tank, like a lone Oscar or Angel. You could try getting another to take its attention away, and this works in many cases.
For the fish that are getting bullied, try additional planting, more places for them to hide in the way of rock arrangements with hiding places that the bullying fish can't get into. As a last resort your LFS (Local Fish Store) might take the large fish in trade for some smaller ones that would be less problematic.
You should not confuse breeding actions with bullying, as most fish tend to look and act aggressive at breeding time. In the case of the Livebearers the males will constantly harass the females, at times leaving the female quite exhausted. Nothing much can be done here as this is nature's way, but if the females become stressed at all you could try separating them from the males for a while, preferably till they have given birth. Returning to the Cichlids for a moment. Many of these species will lock jaws and seem as if they are going to rip each other apart, and this can become quite scary at times, and others will chase their mate around the tank as if they were going to kill it, but in many cases it is just a normal breeding pattern.
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