Aggression.
Lionfish aren't aggressive. Their diet includes other fish. That's not aggressive. That's dinner. How do you manage that? Figure the max mouth size (it can be amazing) and never put something in the tank that's smaller-than unless it is meant to be dinner.
Aggressive actually tends to mean 'territorial.' Will not appreciate other fish in its territory. How this is defined varies with the species. To test this, without getting a fish injured or killed, hold up a mirror. If the fish insanely attacks the mirror, figure getting it a fishy 'friend' of the same species is not a good idea. (This method can sometimes SO distract a fish you can slip up from behind with a net.) Mirrors, however, don't help when you have a fish that is oblivious to territory (mandarins, chromis, eg) or one that has other demands for a certain kind of feature (chromis, eg, will wander through everybody's territories by day, but will fight to the death over sleeping holes at dusk, which is why they disappear one by one until they all have enough holes).
Water conditions or tank shape can also affect this. If your alkalinity is low (I keep mine at 8.3) you will see more aggression: fish aren't comfortable, their skins are not, and they're mad at everything. A tank shape that allows a fish to retreat can bring peace. One that doesn't give enough running-room or caves can cause fights. [Add low alkalinity to that and it's WWIII.]
Fish can also grow INTO aggression, meaning as they mature and become sexual, they can go from cute to killer. One young Clarkii clown can take, oh, about a square foot of tank. A mated pair wants about half a 100 gallon tank.
Then there are the freakouts, that aren't so much aggression as a nervous breakdown. A 2" rabbitfish that was mildly freaky in a 50 can start killing tankmates when he tops 3", not because he's attacking, but because he freaks and panics (with a lethal top spine) when a tankmate accidentally surprises him in a corner. A tang that was fine in a 50 at 2" can start hanging in one spot at 3", dithering, ending in a freakout that takes the fins off several fishes and again, involves a dangerous tail-spike. Fish do not stop growing. They get bigger, and ultimately, if they outgrow their tank, they can get get fat instead of long (this is not healthy) and sometimes suffer an apparent meltdown, maybe triggered by low alk, maybe just a longstanding fishy spat gone nuclear option.
Planning a tank means planning ahead: fish grow. As newcomers to the hobby, it's hard to fit all these puzzle-pieces comfortably into a tank, but asking is good, believing the adult size estimates on online sites is very good: and here's where you need to use smarts. The seller will tell you a 'minimum tank requirement' that is the very least tank in which this species will live if they sell you this fish, which they'd love to do: it is, however, somewhat a subjective guess and doesn't include tank shape---but they'll also tell you (or you can look up on Google) the adult size of the fish. Go by the adult size: that one is objective truth. Also consider its life habit. If Google says it swims two miles a day in the wild, a corner tank is not going to be good for this fish.
Lionfish aren't aggressive. Their diet includes other fish. That's not aggressive. That's dinner. How do you manage that? Figure the max mouth size (it can be amazing) and never put something in the tank that's smaller-than unless it is meant to be dinner.
Aggressive actually tends to mean 'territorial.' Will not appreciate other fish in its territory. How this is defined varies with the species. To test this, without getting a fish injured or killed, hold up a mirror. If the fish insanely attacks the mirror, figure getting it a fishy 'friend' of the same species is not a good idea. (This method can sometimes SO distract a fish you can slip up from behind with a net.) Mirrors, however, don't help when you have a fish that is oblivious to territory (mandarins, chromis, eg) or one that has other demands for a certain kind of feature (chromis, eg, will wander through everybody's territories by day, but will fight to the death over sleeping holes at dusk, which is why they disappear one by one until they all have enough holes).
Water conditions or tank shape can also affect this. If your alkalinity is low (I keep mine at 8.3) you will see more aggression: fish aren't comfortable, their skins are not, and they're mad at everything. A tank shape that allows a fish to retreat can bring peace. One that doesn't give enough running-room or caves can cause fights. [Add low alkalinity to that and it's WWIII.]
Fish can also grow INTO aggression, meaning as they mature and become sexual, they can go from cute to killer. One young Clarkii clown can take, oh, about a square foot of tank. A mated pair wants about half a 100 gallon tank.
Then there are the freakouts, that aren't so much aggression as a nervous breakdown. A 2" rabbitfish that was mildly freaky in a 50 can start killing tankmates when he tops 3", not because he's attacking, but because he freaks and panics (with a lethal top spine) when a tankmate accidentally surprises him in a corner. A tang that was fine in a 50 at 2" can start hanging in one spot at 3", dithering, ending in a freakout that takes the fins off several fishes and again, involves a dangerous tail-spike. Fish do not stop growing. They get bigger, and ultimately, if they outgrow their tank, they can get get fat instead of long (this is not healthy) and sometimes suffer an apparent meltdown, maybe triggered by low alk, maybe just a longstanding fishy spat gone nuclear option.
Planning a tank means planning ahead: fish grow. As newcomers to the hobby, it's hard to fit all these puzzle-pieces comfortably into a tank, but asking is good, believing the adult size estimates on online sites is very good: and here's where you need to use smarts. The seller will tell you a 'minimum tank requirement' that is the very least tank in which this species will live if they sell you this fish, which they'd love to do: it is, however, somewhat a subjective guess and doesn't include tank shape---but they'll also tell you (or you can look up on Google) the adult size of the fish. Go by the adult size: that one is objective truth. Also consider its life habit. If Google says it swims two miles a day in the wild, a corner tank is not going to be good for this fish.
Last edited:
