Critical mass for schooling

  • Get the NEW AquariaCentral iOS app --> // Android version will be out soon!


AC Members
Dec 10, 2005

What is the minimum number of cories to get good schooling behavior? I want a small group in a 75 gallon tank. Would 6 be enough? I know the tank is big, but I want to keep discus and cardinals too and I don't want the tank to be heavily stocked...

Also, is carab sea Tahitian moon sand a good substrate?



Byron Amazonas

AC Members
Jul 22, 2013
Pitt Meadows (within Greater Vancouver, BC) Canada
Real Name
Five is the minimum number of corys to be kept in an aquarium, according to the authoritative sources. This has more to do with their stress level than anything else. But everyone will agree that the more the better. If I am thinking of the same aquarium from another thread, this is a 70 or 75g tank, and 12 were mentioned previously; I would go with that. The corys will be much happier, and more interesting for you.

Some aquarists stay with one species, others (like me) mix the species. This is a matter of personal choice, unless one intends spawning, in which case species should not be mixed. Doing this over 15-20 years I have observed that some species tend to remain much more together than others. Whenever there is any annoyance/disruption, such as moving to a new tank, almost all species will remain close together as a species for anywhere from a few days to a few weeks. The point here is that the number does impact the fish's level of stress, and they will always be less stressed if they know there are more of them present.

Some of my species have only one or two, usually because I bought the last ones in the store and haven't found more. When I can, I try to get 5 of a species. I presently have just over 30 in my 115g, representing about 12 species. The C.panda are almost always together, or close; C. duplicareus tend to stay near each other too, though they also seem to really enjoy playing with the C. sterbai; two fish, one of each species, will often chase each other around the tank, one leading and then the other, nudging each other and caressing with their barbels. I have a group of five C. schultzi in another tank, and they remain fairly close almost all the time; now and then I will see two in one spot, and three elsewhere, but usually not for very long.

Tank space and decor also impacts upon shoaling (schooling) behaviour. Water flow can as well, and of course the other tank inhabitants. Floating plants will calm them more than an open surface.

I can't answer your sand queston as I have never used those products; I use Quikrete Play Sand which happens to be near-identical to many creek sans in Amazonia, and my corys certainly love sifting through this. Spawning is regular too, though few fry survive in a community setting.

  • Like
Reactions: Lady JinglyJones


Josh Holloway--Be mine!!!
Sep 1, 2003
Calif. SF Bay area
Visit site
Real Name
Hello Nate:

I have that sand on top of my ECO, but because it is smaller, it has (over the years) sunk into the ECO.

I have corys over it without a problem. It is fiarly fine, and did not hurt my corys.
You said the 75 is big, trust me they will find each other. My tank (with the black sand) is a 120, and they find each other just fine.

Also you want Discus and Cardinals, both these fishes like warmer water (about 80), and the Cory Sterbai would be a very good mix. :)


AC Members
Aug 29, 2000
North-Central PA
Real Name
Yeah, not many cories like it warm. Sterbai's are usually the go to species (and for good reason!!) but there are some others - just do your research