What is the rational behind keeping odd number of certain fish?

[Tripod1404]

It is a common advice to keep certain schooling and hierarchical fish like anthias or green chromis in groups of odd numbers.

What is the advantage of keeping 7 chromis compared 6 (aside from one extra chromis to deter aggression of course). Or like; Would 7 chromis be better than 8 chromis?

IME for green chromis best number is 1, 2, or 10+. Otherwise there is constant struggle between a small number of fish. I was able to have long term success with a pair and in the past with a school of 20+. But any other number never worked.

For Anthias, it is specie specific and it also depends on the male anthias's ability to keep the females as females. But generally 4 females per male works

[der_wille_zur_macht]

The rationale is that it makes the person saying it sound like they're smart.

I've never seen any real evidence to suggest fish understand what odd numbers are.

[HBtank]

It only matters between 1, 2 and 3; going from 2 to 3 can disperse aggression enough to make a situation tolerable, and sticking with a single fish instead of a pair works almost universally. Beyond that odd/even has never made sense.

[oldhead]

Feng Shui

[Tripod1404]

Originally Posted by HBtank

It only matters between 1, 2 and 3; going from 2 to 3 can disperse aggression enough to make a situation tolerable, and sticking with a single fish instead of a pair works almost universally. Beyond that odd/even has never made sense.

I agree with this for anthias that the hierarchy is mainly gender based. Male anthias is the most aggressive and the dominant. There is also female to female aggression but that is low as long as there is a male. And normaly you should only have one male in the mix. So one male with 2 females divide the aggression to 2.

But in chromis hierarchy is mainly based on size (males as slightly bigger but a large female can still be bigger than a small male), so the bigger fish try to dominate the smaller ones. Unlike anthias where you can have a single male, you cannot have 3-4 chromis where the is one big chromis. There will always be certain size difference between them. So like there will be biggest, bigger, big and not big (aka punch bag) . So in chromis always had better track record with 2 chromis over 3 (or for that matter even till total number us at least 8).

When I had 3, a linear hierarchy of "1st-2nd-3rd" forms and the least dominant 3rd fish gets beaten up by 1st and the 2nd. In addition to this, since the 2nd fish shows domination behavior and colors towards the 3rd fish, these kinds of dominant behavior provokes the 1st fish to beat the 2nd fish.With just two, you end up with one dominant and one submissive fish. Since the submissive fish doesn't have a 3rd fish to exert its dominance, it doesn't show dominant behavior or colors, so the dominant fish doesn't constantly try to pressure it.

When you add more chromis, this situation gets even worse because again a linear hierarchy is established but this time there are more partially dominant fish in the middle. To make things worse, some fish try to move up in hierarchy by fighting and if that happens, the whole group try to establish a new hierarchy again. This cause constant squabbling.

However, when there is more than 10, they can no longer form a strictly linear hierarchy and there is too many fish divide up the aggression and provide diistaraction. It gets more unlikely that two fish that dont get along always founds themselves close to one and other.

[ca1ore]

Yeah, I think it's weak at best. With tangs, 3 is clearly better than 2 as a way to diffuse aggression, but 4 is then better than 3, and so on.

[nereefpat]

Originally Posted by Tripod1404

But in chromis hierarchy is mainly based on size (males as slightly bigger but a large female can still be bigger than a small male), so the bigger fish try to dominate the smaller ones. Unlike anthias where you can have a single male, you cannot have 3-4 chromis where the is one big chromis. There will always be certain size difference between them. So like there will be biggest, bigger, big and not big (aka punch bag) . So in chromis always had better track record with 2 chromis over 3 (or for that matter even till total number us at least 8).

When I had 3, a linear hierarchy of "1st-2nd-3rd" forms and the least dominant 3rd fish gets beaten up by 1st and the 2nd. In addition to this, since the 2nd fish shows domination behavior and colors towards the 3rd fish, these kinds of dominant behavior provokes the 1st fish to beat the 2nd fish.With just two, you end up with one dominant and one submissive fish. Since the submissive fish doesn't have a 3rd fish to exert its dominance, it doesn't show dominant behavior or colors, so the dominant fish doesn't constantly try to pressure it.

When you add more chromis, this situation gets even worse because again a linear hierarchy is established but this time there are more partially dominant fish in the middle. To make things worse, some fish try to move up in hierarchy by fighting and if that happens, the whole group try to establish a new hierarchy again. This cause constant squabbling.

However, when there is more than 10, they can no longer form a strictly linear hierarchy and there is too many fish divide up the aggression and provide diistaraction. It gets more unlikely that two fish that dont get along always founds themselves close to one and other.

This is a really interesting take on chromis. I think it's a good possible explanation for their poor group survival ranks in our tanks.

[BigDave]

Originally Posted by oldhead

Feng Shui

My understanding is there's a lot of truth in this. In art you see the use of thirds a lot, and most things that are 'pleasing to the eye' are done in odd numbers. In gardening you typically add plants in odd numbers too.

The same can be somewhat said for schooling fish. Now... once you get to larger numbers, your eyes can't really discern if it's even or odd anymore, so it matters a lot less.

The other argument about 1&2 vs 3 is also true. 1 is good. 3 is good. 2 and the larger one will typically dominate the smaller. Once you get past 3, they all typically chase each other and it matters a lot less.

[ntropics]

I only keep prime numbers of fish. 1,2,3,5,7,11 etc...

[rffanat1c]

I prefer to keep 3.14159265359 number of fish...

[oldhead]

Originally Posted by rffanat1c

I prefer to keep 3.14159265359 number of fish...

Is that 3 normal sized fish and one dwarf?

[ReefkeeperZ]

Originally Posted by Tripod1404

I agree with this for anthias that the hierarchy is mainly gender based. Male anthias is the most aggressive and the dominant. There is also female to female aggression but that is low as long as there is a male. And normaly you should only have one male in the mix. So one male with 2 females divide the aggression to 2.

But in chromis hierarchy is mainly based on size (males as slightly bigger but a large female can still be bigger than a small male), so the bigger fish try to dominate the smaller ones. Unlike anthias where you can have a single male, you cannot have 3-4 chromis where the is one big chromis. There will always be certain size difference between them. So like there will be biggest, bigger, big and not big (aka punch bag) . So in chromis always had better track record with 2 chromis over 3 (or for that matter even till total number us at least 8).

When I had 3, a linear hierarchy of "1st-2nd-3rd" forms and the least dominant 3rd fish gets beaten up by 1st and the 2nd. In addition to this, since the 2nd fish shows domination behavior and colors towards the 3rd fish, these kinds of dominant behavior provokes the 1st fish to beat the 2nd fish.With just two, you end up with one dominant and one submissive fish. Since the submissive fish doesn't have a 3rd fish to exert its dominance, it doesn't show dominant behavior or colors, so the dominant fish doesn't constantly try to pressure it.

When you add more chromis, this situation gets even worse because again a linear hierarchy is established but this time there are more partially dominant fish in the middle. To make things worse, some fish try to move up in hierarchy by fighting and if that happens, the whole group try to establish a new hierarchy again. This cause constant squabbling.

However, when there is more than 10, they can no longer form a strictly linear hierarchy and there is too many fish divide up the aggression and provide diistaraction. It gets more unlikely that two fish that dont get along always founds themselves close to one and other.

I concur, when you look at chromis in the wild and the numbers that tend to school together you'll see higher numbers are definitely better than a tiny school anywhere from 3-12 your constantly losing the little ones you start going uphill of that and the aggression is disbursed.

a side note on the survival of chromis, they feed all day long similar to anthias so multiple feedings a day helps reduce aggression and keeps them stronger as well as a structure similar to a very large branching coral like a +14" acropora colony provides an appropriate structure that they would inhabit in the wild giving proper shelter to the school allowing the smallest to retreat easier. most of out tanks even coral heavy tanks dont have the right type of structure for them.

[Hal]

I've tried several group sizes of chromis, with 7-8ish being my largest. In every case, the group eventually dwindled to one in a fairly short time period (less than a year).

[Tripod1404]

Originally Posted by Hal

I've tried several group sizes of chromis, with 7-8ish being my largest. In every case, the group eventually dwindled to one in a fairly short time period (less than a year).

About 20 years ago my dad kept more than 20 in a ~200 gallon tank. As far as I remember their numbers were pretty stable trough out my childhood. He still has 20+ but of course not the original fish, he added more as they started to die from old age. But most easily lived 10 years. He still has several huge ones, they might be the ones that he bought 20 years ago .

I had long term success with 8 (started with 10). They seem to do better if the tank has strong flow, there was more fighting when I turned the pumps for cleaning or feeding corals.

Also adding another type of open water column fish (ideally another planktivore) that is more aggressive than them helps, this prevents any chromis from becoming the most aggressive/dominant fish in the water column. Imo best option is lyretail anthias, it is relatively hardy, cheap, available, accepts most prepared foods and doesn't need to be fed as often as some other anthias. I had a group of 6 (1 male 5 female). Male anthias absolutely dominated the chromis and even females didnt take it kindly to chromis that strayed too close. So the chromis were more busy with keeping their distance from the anthias (and dodging occasional dives of the male anthias) than fighting among themselves. It even induced some degree of schooling during periods the male was especially territorial and aggressive. Plus it reduced the male aggression on female anthias.

~3 years ago I gave the chromis to open space for more "challenging" anthias. But the 4 of the 6 lyretail anthias still remain. And male is still an a**hole to other planktivore.

[rogersb]

Originally Posted by rffanat1c

I prefer to keep 3.14159265359 number of fish...

At least contribute something rational to the conversation...

[rffanat1c]

Originally Posted by rogersb

At least contribute something rational to the conversation...

It was very rational scientific humor. And specific.


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[protie]

Originally Posted by Tripod1404

I agree with this for anthias that the hierarchy is mainly gender based. Male anthias is the most aggressive and the dominant. There is also female to female aggression but that is low as long as there is a male. And normaly you should only have one male in the mix. So one male with 2 females divide the aggression to 2.

But in chromis hierarchy is mainly based on size (males as slightly bigger but a large female can still be bigger than a small male), so the bigger fish try to dominate the smaller ones. Unlike anthias where you can have a single male, you cannot have 3-4 chromis where the is one big chromis. There will always be certain size difference between them. So like there will be biggest, bigger, big and not big (aka punch bag) . So in chromis always had better track record with 2 chromis over 3 (or for that matter even till total number us at least 8).

When I had 3, a linear hierarchy of "1st-2nd-3rd" forms and the least dominant 3rd fish gets beaten up by 1st and the 2nd. In addition to this, since the 2nd fish shows domination behavior and colors towards the 3rd fish, these kinds of dominant behavior provokes the 1st fish to beat the 2nd fish.With just two, you end up with one dominant and one submissive fish. Since the submissive fish doesn't have a 3rd fish to exert its dominance, it doesn't show dominant behavior or colors, so the dominant fish doesn't constantly try to pressure it.

When you add more chromis, this situation gets even worse because again a linear hierarchy is established but this time there are more partially dominant fish in the middle. To make things worse, some fish try to move up in hierarchy by fighting and if that happens, the whole group try to establish a new hierarchy again. This cause constant squabbling.

However, when there is more than 10, they can no longer form a strictly linear hierarchy and there is too many fish divide up the aggression and provide diistaraction. It gets more unlikely that two fish that dont get along always founds themselves close to one and other.

Very interesting post, thanks. Since you mentioned a few fish in my tank I thought I would post my observations of aggression.
I have 2 small Yellowtail Damsels which are constantly chased back into the rocks by the 2 larger Blue Damsels which are constantly chased by my large Pink Square Anthias. This has gone on for about 9 months now.

[der_wille_zur_macht]

Interesting discussion on Chromis groups and I'm happy that it's gone beyond the usual "it works!" "no it doesn't!" bickering.

I've kept groups of chromis many times in many tanks. Sometimes it's been smaller than the magic numbers mentioned here, but only in very large tanks. I accept the logic presented here, and I'm wondering if this forms a second "how to get it to work" argument - small groups can work, in large tanks. Medium sized tanks need the larger number per the above post.

I recently tried to keep 3 in a small tank and I had my first ever chromis failure. Two of them are now living in (separate) sump compartments, they got there on their own by jumping into the overflow trying to avoid fights with the dominant fish. The dominant fish is still in the display. In 25ish years of reefkeeping, this is the first time I've had significant aggression.

[Davidg231]

My understanding is that the dominant fish just has more fish to beat up and spread the love. Although I had three yellow tangs and now I have two that don’t mind each other and don’t beat each other up. Maybe they paired?!?

[der_wille_zur_macht]

Originally Posted by Davidg231

My understanding is that the dominant fish just has more fish to beat up and spread the love.

I'm not sure it's safe to make sweeping statements across all species of fish - there are probably lots of different models of social behavior in different types.

[ktownhero]

Something about an even number of fish doesn't look right to me, in sizes where you can see it. I have 4 green chromis and it annoys me lol.

[der_wille_zur_macht]

Originally Posted by rogersb

At least contribute something rational to the conversation...

It was an irrational answer but at least it wasn't imaginary.

[NS Mike D]

odd grouping are eye pleasing. 2 different professional gardeners/landscapers told me this explaining their lay outs. I have to admit, I do find groupings of 3 more appealing to 2 or 4 all over the place (gardens, fish tanks, nick knacks etc)

[der_wille_zur_macht]

I think that's definitely true, at least in static designs (like a garden layout). Not sure if it applies as strongly in a fish tank where the things you're trying to subconsciously count are moving around anyways. Even in a small tank - I have an even number of fish in my 24g cube but you pretty much never see them all at the same time.

[Tripod1404]

Originally Posted by der_wille_zur_macht

Interesting discussion on Chromis groups and I'm happy that it's gone beyond the usual "it works!" "no it doesn't!" bickering.

I've kept groups of chromis many times in many tanks. Sometimes it's been smaller than the magic numbers mentioned here, but only in very large tanks. I accept the logic presented here, and I'm wondering if this forms a second "how to get it to work" argument - small groups can work, in large tanks. Medium sized tanks need the larger number per the above post.

I recently tried to keep 3 in a small tank and I had my first ever chromis failure. Two of them are now living in (separate) sump compartments, they got there on their own by jumping into the overflow trying to avoid fights with the dominant fish. The dominant fish is still in the display. In 25ish years of reefkeeping, this is the first time I've had significant aggression.

I had success keeping two in 40 breeder (and for some time in 20g for qt),but one was 1.5 times larger than the other when i first got them and they came from separate tanks of the lfs. So they saw each other for the first time in my tank and the size difference pretty much dictated the hierarchy without any fights. As of today the size difference is smaller but they dont fight, there is occasional chasing but that is all. And based on my success with mixed schools, I keep them with a single male lyretail anthias.

I think with a pair, in damsels, it is better to have a certain size difference. This was the smaller fish submits without fighting. When these fish fight, whether chromis, clown or bluedevil, they tend to really hurt each other a lot (like missing scales, riped fins etc.) and than get 2ndry infections and etc.

But with a pair of chromis, it is also worth mentioning, the sex of the fish might dictate long term success. A male and a female might work better than two females or definitely two males. Unfortunately, it is hard to determine the sex of these fish.