if there are "ideal" group sizes... why is that population level never maintained???

[Magnum Man]

I know I'm not the only one this happens to, but you are looking at a schooling fish, maybe you even buy a handful extra, only to have the survival rate drop to below the "ideal" you may have a strong group, but at least for me, by the time things shake out, I end up with a less than"ideal" number of fish... seems that ideal number is unsupported by facts, as the smaller group thrives... I've even bought twice as many, and ended up with a stable population, always less than that "ideal" number???

maybe the stable population is more related to the size of the tank??? most of my tanks are not "small", so this kind of frustrates me...

 

[GaryE]

It relates not only to tank size, but to crowding. Stress kills fish, especially fish for whom a shoal is a defensive group.

Three of my tanks I find most interesting at the moment are a homemade 28.5 inch x 15 x 15 tank with 10 Carnagiella strigata hatchets and 9 Hoplisoma atropersonatus - fish that stay out of each others' way, a standard 20 tank with 10 Carnagiella myersi, a tiny, shy fish I wouldn't expect to see often in a community (but see all the time in a single species set up), and a 20 tall with 10 Nannostomus marginatum - absolutely confident in a jungle of plants with no other species.

While this can't (and shouldn't) be proven, I'll wager I would have lost a few from each group if they were in a community tank. I had three of the Hoplisoma, and added six. The other fish were all bought in groups of 10, and all survived shipping for a good distance. They're well past QT now, but they will stay in the set ups I have. More and more, I'm finding tetras thrive in good sized tanks with no other tetra species. A corydoras type or two doesn't seem to matter. I have Hyphessobrycon melanostichos in a 4 foot 40 long with 10 Hoplisoma grantii and 9 concolor, and not one fish has died since arrival. In the tank directly below 6 H. margitae have rapidly grown in with 2 cory group gaggles - some brevirostre, and loretoensis.

I do seem to lose tetras in my 6 footer, where there's a community that includes some mildly territorial tetras. I'm thinking that to manage that, I need to stop thinking individuals, cycling/bioload or combinations, and start looking counting species. With my arithmetic skills, it's easy. One.

 

[Innesfan]

I suspect there are also some ideal shoal sizes that we don't know about for some species. A favorite of mine, Pethia setnai, which when in full color are very similar to the old favorite P. reval, alias P. cumingii (Cuming's Barb) is found in the wild in shoals of 8 or 9 lorded over by a dominant male. This detail was included in a scientific paper on the species and is the sort of valuable information that we seldom hear about. Sure enough, if I keep fewer than 8, there's aggression that could become lethal if I do not intervene. And when there's more than 10 there are a few that never emerge from the plant cover. The current shoal of 9 in a 75g have been out front and stable from day one. Without the corroborating report about the species in the wild, my experience might be considered anecdotal; but together with that report it sure suggests that for this species 8 to 9 might well be the Goldilocks number in the wild and in the aquarium.

 

[GaryE]

@Innesfan - intriguing.

I was able to watch many hours of raw, unedited Amazon footage, several times at a fish explorer friend's house. I was stuck by a few types of Characin shoals. There were the ones with under ten fish, that were often within merging range of several other groups. You couldn't call them one big shoal, and they were more like foraging groups. But I bet if the gopro sitting on the bottom had suddenly been moved, they would have formed one defensive group out of six or seven smaller gaggles foraging along.

Fish from the cory group did the same thing, from what I could see.

There were others that cruised in about 30 or more, often fish higher in the water column, and often silvery ones.

And finally there were the shoals of hundreds or thousands that looked like shots from a reef. They weren't that common in the habitats my friend filmed in. Rummy nose types though - there were lots.

At various points, there were mixes, smaller groups, larger groups, scattered individuals, but patterns like above. Often when you'd see shoaling, you'd soon see Crenicichla shoals arrive, or large lone predators flash by.

Meanwhile we buy them in sixes, because that's how we buy cupcakes or muffins. The late Byron cited a study that showed ten to be a basic minimum number for shoalers, but that was an attempt to create a minimum because of the size of our tanks. Why not 20? or 30? or 100?

The answers to me are space and budget.

We're lucky now to have increasing access to video about fish in the wild. But because of the idiocy factor online, if you put up a longer video, no one watches it. Attention spans are really short. So everyone edits their videos down to basics, and while we get to see highlights, we miss a lot too.

 

[Innesfan]

My example was for a species of Asian barb which generally are kept in smaller shoals than New World characins. For the latter, I have found that the rhomboid shaped tetras--the Rosy clade, Lemons, Garnets, Black, Von Rios, Head-and-tail Lights, etc,-- are more likely to be lethally fractious in numbers fewer than ten or so and over time will pick each other off to the last man standing if the numbers dwindle down from double digits; while the torpedo shaped tetras--the Neon clade, Glowlights, the three Rummies, Loretos, etc-- will be fine if their numbers dwindle down to 5 or 6, though they look best and are most active, IMO, if kept in much larger schools.

I too have been fascinated by those in situ videos available on line, especially the lengthy ones. As a Nannostomus fan, it's always fun to see a few N. digrammus or N. marilynae interlopers among a swarm of Green Neons or Cardinals.