...is a sign you have too many fish, too small a tank, or inappropriately mixed species.
Territory defense and chasing is business-as-usual for some species. If there is no injury to the intruder and the chase stops, no big deal. That's just chasing the neighbor lad off the lawn.
Pieces out of fins and wounds are another matter. This leads to dead fish, diseased fish, and a failing tank.
What's enough room? Size of the fish is NO indicator of how much room they need. If the swimming is easy and the fish reaches a turn-back on his own without running into something, he's at ease and has enough territory. If he runs into glass or gets chased off by a neighbor, that's to watch. You may just have overstocked. Fin-flicks, direction changes like a guy standing on a street corner and wondering which way to go, OR aggression, fin-display, nervous twitches, all are indication of a fish feeling confined. This can lead to a decline in health of the fish in question; and can also indicate your tank isn't producing enough oxygen --- which can kill everybody on a too-warm day (warm water loses its ability to 'carry' oxygen) ---
Certain species are perfectly fine until crowded. Clowns, other damsels (the clowns are damsels,) gobies, blennies, angels, tangs--especially; rabbitfish---all these are great mixed-tank fish, until you either put in a rival, or get too many fish in too little space. Then fish will mysteriously disappear, one by one, fins will get ripped, and it's just not fun.
Aggression is a panic reaction: it's fear, and discomfort, and it's going bad places. If you're seeing this in your tank, you've got a problem, and you need to thin the herd, check on tank size requirements, and make sure your rock arrangement has a sleeping hole for every fish.
Territory defense and chasing is business-as-usual for some species. If there is no injury to the intruder and the chase stops, no big deal. That's just chasing the neighbor lad off the lawn.
Pieces out of fins and wounds are another matter. This leads to dead fish, diseased fish, and a failing tank.
What's enough room? Size of the fish is NO indicator of how much room they need. If the swimming is easy and the fish reaches a turn-back on his own without running into something, he's at ease and has enough territory. If he runs into glass or gets chased off by a neighbor, that's to watch. You may just have overstocked. Fin-flicks, direction changes like a guy standing on a street corner and wondering which way to go, OR aggression, fin-display, nervous twitches, all are indication of a fish feeling confined. This can lead to a decline in health of the fish in question; and can also indicate your tank isn't producing enough oxygen --- which can kill everybody on a too-warm day (warm water loses its ability to 'carry' oxygen) ---
Certain species are perfectly fine until crowded. Clowns, other damsels (the clowns are damsels,) gobies, blennies, angels, tangs--especially; rabbitfish---all these are great mixed-tank fish, until you either put in a rival, or get too many fish in too little space. Then fish will mysteriously disappear, one by one, fins will get ripped, and it's just not fun.
Aggression is a panic reaction: it's fear, and discomfort, and it's going bad places. If you're seeing this in your tank, you've got a problem, and you need to thin the herd, check on tank size requirements, and make sure your rock arrangement has a sleeping hole for every fish.