Aggression in normally peaceful fish

[droog]

Hi,

Newbie question: what would cause a normally peaceful fish exhibit occasional aggressive behaviour? Is hunger a likely factor? Water parameters? Territory requirements as fish grow? If its a combination, which are most likely to be important?

The reason I ask is that I have a community of Cardinalfish in my 120g (5 long spine cardinals and 7 pajama cardinals). They like to hang out together in my tank and have been very peaceful for a year. But they have grown quite a bit during that time. Recently I have noticed occasional aggressive behaviour between them, a quick chase away or peck. Its not bad or constant nagging (like damsels!) but they were sooooo peaceful before.

I feed twice a day, usually pellet once and frozen brine shrimp once. I also had a float valve malfunction over the past few weeks that was causing inconsistent additions of Kalk into to my sump. I believe this caused a salinity drop from 1.026 to 1.022 (slowly but surely) over a 24hour period.

-droog

[dkeller_nc]

It may simply be that your cardinalfish are establishing social dominance in the school as they reach sexual maturity from juveniles. Live Aquaria's entry for both species notes that they can be maintained in schools without aggressiveness (unlike other cardinalfish, such as bangaiis). So long as your tank's large enough to accommodate the size of the school that you have, occasional pecks/chases shouldn't cause any issues.

Just as an example, I have 4 longspine/threadfin/blue-eye cardinals (all synonyms for the same fish - Zoramia leptacantha) in a 50 gallon cube. This seems to be about right as far as number of fish/swimming room.

[Sk8r]

Tank size matters. People howl about the ferocity of damsels. I like the types and have a 100 gallon tank with damsels, gobies and blennies---no problems.

Species habit matters. A mandarin, the most peaceable of fish, will kill another mandy if it shows up in an area it calls its territory---and a mandy can claim about 50 gallons or so of territory.

Yellow watchman goby---will kill another of the same gender if in its territory.

I've heard that cardinals, ordinarily peaceful, can get cranky about space requirements for their group.

[cloak]

Maybe they're getting old.. Yelling at the TV sometimes. ;)

[tmz]

Bangais(Pterapogen kauderni) will kill off members of the same sex or competing pairs even in larger tanks; 120 gallon IME Tank size might give enough territory to prevent that in some cases.
Many of the other types(Apogon and Sphaeramia/including pajamas) tend to be ok in schools ,generally. Though I suppose they all can become territorial and cantankerous over time particularly without cable tv.