so, what is the real reason for Cory's shoaling???

[Magnum Man]

I have 2 fairly large shoals of Cory's in 2 different tanks... one is a medium size South American Cichlid tank, with relatively peaceful Cichlids... the Cory's all hang in a group, fat and happy, and typically one or two at a time venture out from their group, they obviously don't need the protection of a shoal, yet are almost always in a tight group and have no respect for anyone else trying to set up turf, even when out of the group, as a single...not showing any fear, venturing out as singles... this tank is around 78 degrees...

both groups are aeneus, 1st ones are albino, the 2nd group are long fin bronze...

the 2nd shoal is in my barb tank... there are 10 inch Tin Foil barbs as the biggest fish, in this tank, and the Temperature is set at 72 degrees... these guys venture all over the tank in singles or pairs, and never gather in a group, almost always spread out over the whole bottom of the tank ( they may sleep together, but my light begins to slowly come on, before I wake up, so I really don't witness these in total darkness ) ... these work the sand, leaving patterns in the sands surface, and appear to be thriving, but don't seem to shoal at all, or show intimidation at all from the bigger fish???
I listed the tank temps, just in case that would effect their natural behavior...

 

[GaryE]

I've long avoided ever keeping Corys with medium sized Cichlids. They need the same part of the tank, so they will clash. Or, to be more exact, the Cichlids will threaten the Corys, who will band together defensively. There may not be actual attacks, but the Corys see the territoriality. So they adopt the sensible behaviour, and band together. It isn't a reaction to the size of the tankmates, but to behaviour.

While substrate oriented Cichlids will treat Corys as dangers to their eventual breeding grounds, midwater barbs couldn't care less if the Corys live or die. They aren't their concern. So the Corys can forage in smaller groups, a totally uninteresting creatures to the barbs. The size doesn't matter if the behaviour isn't predatory or territorial.

 

[CassCats]

Barbs are more upper level fish while cichlids tend to be more lower substrate level. Cories are sorta "dumb" in the way that they don't understand territory lines, and it's plausible that they do feel some tension from the cichlids and tend to feel safer in a larger shoal, where as the ones with the barbs don't have that worry of crossing into a territory so are more relaxed to explore.

 

[GaryE]

I don't think the Corys are dumb about territory. I read a serious study that said the territory of a wild krib (pelvicachromis pulcher) can be 9 square meters. Based on that, I figure I have never seen a home aquarium large enough to provide territory for any Cichlid that claims turf. They adjust to cramped space and take smaller territories, but every bit of a home tank with Cichlids "belongs" to those Cichlids. Corys have no choices but to cross boundary lines.

I just hope they're unaware enough not to take it as permanent stress.

 

[Magnum Man]

they have no issues swimming out and around by themselves, and I've never witnessed any physical interaction between any of the Cichlids...
I understand there are subtle interactions... the last angel splitting I had to do, involved the dominant fish just "mean mugging" the other, into submission... and no real physical interactions

 

[MaloK]

They act like that as long as they feel there's a complete absence of any fear and menace.

In a tetra and pygmy cory tank, nobody schooled until a dwarf gourami appeared. The next day everybody was in tight rank in their place.

I believe once they start, the behavior will be nearly permanent until the menace is removed and enough water has been changed to remove trace of communication hormones.

 

[GaryE]

I've often wondered how defined fears are. Would a Cory react differently to a Cichlid that would be a danger in its ancestral habitat? Are details instinctively recognized?

 

[Magnum Man]

... and how much perceived "danger" would it take to release toxins???

 

[GaryE]

It wouldn't take much. Any threat. In the wild, it's diluted immediately, and just informs other Corys to keep an eye out for possible trouble. It's the chemical equivalent of "heads up".

It can build up in the closed confines of a tank. I've had fish get frightened (not just Corys) and remain so until I did large water changes.

 

[MaloK]

Yes, I also tried activated carbon and it didn't work, only water changes brought calm back.

I had this phenomenon in dwarf powder blue gouramies breeding tanks. And it was caused by the light coming on in the morning. The freakout amplified until shade where added and nervosity went down after all water was changed.

 

[fishorama]

For me, corys of many species, don't understand or care about the entire territory thing. Not just with cichlids, but with loaches, "sharks" (red tailed & rainbows) & plecos too. If there's food involved, dang the danger, full steam ahead!!

 

[GaryE]

A small point about the toxin. It's in some species only in its toxic form, but all have communication chemicals. Some just happen to be toxic when concentrated. Most aren't, but they still hit the fight or flight system with flight, in the case of peaceful Corys. It's a toxin, not a venom, and it isn't evolved to kill enemies. It just so happens to harm the Corys themselves too - not in nature, but in the tiny bodies of water we keep them in. Nothing in their natural history prepared them for 30 gallons of rarely changed water. They live in a constant flow of fresh water, in a rainforest zone. We exploit their adaptations for the dry season, when things get temporarily difficult for them, and conditions approach those of a well cared for aquarium.

Niche is really important when designing an aquarium. I like substrate oriented species a lot, but am very limited in which ones I can keep. True, we stock our aquariums for ourselves and tend not to care about the needs of the fish, but over time and with experience, I got sick of doing that. I meant no harm to the fish, but I was putting them in lousy set ups.

Now, all my Cory types are the only fish on the bottom of their tanks. I have 2 tanks where 2 species share the space. In a couple, I have Ancistrus, who have no interest in Corys. I'll never again put Corys in a Cichlid tank, even with dwarf Cichlids. I see what @fishorama mentioned. They have a total disregard for territory. They also tend to sift their ways along the bottom and can cover a fair bit of stream every day. They don't stay in one place like territorial fish do.

We see them in their tiny gaggles of foragers, moving in 2s and 3s and keeping track of where the shoal is. If they encounter danger, they shoal tightly, then go back to foraging. They move along stream bottoms in a flow of Corys. In tanks, the walls stop them. Newly bought wild Corys spend a couple of days glass surfing, til they give up. It's only recently that I understood that behaviour. They're expecting a 30 foot run, and to quickly cross any territories without hanging around and being annoying.

 

[Magnum Man]

so, I'm going to try doing a twice normal water change, on the Cichlid tank, and see if that loosens up the Cory's any??? it may be competition for breeding space ( for the Cichlids, and resulting hormones from them), that are effecting the Cory's???