danger of mixing one of each kind, rather than a school of only one???

Magnum Man · Jul 27, 2026

I seem to be seeing acceptance and not, depending on the fish… in my tetra tank, when the long timers out live their initial school, they often started hanging out with each other ie. my lone cardinal, and lone rummy nose began hanging out… in my silver dollar tank ( it didn’t start out that way ), but there is one each or 4 different kinds , and they appear as if it were only one kind, with the normal pecking order, and no one being ousted from the group….

in a no breeding community aquarium, having one of each kind that belong and act as a group could be more interesting than only one species…

now another week in, and my lone cardinal has joined the newly added group, bringing along the lone rummy nose… this week 6 additional rummy’s are coming… with the loners being buddies. it’ll be interesting if in the long run, the cardinals, and rummy’s go for a co mingled group, to hold tank space against the emperor group that totals a dozen or more…

GaryE · Jul 27, 2026

I'll bet you they won't. Cardinals shoal in the bottom third of the tank by preference, and rummy noses, depending on which species, should like the open water higher up. If they form a shoal because of the threat of the emperor group, then the tank would be seriously out of whack.

Shoalers kept alone are sad things. I don't know how anyone could find them interesting, since they are fish evolved to operate in groups of their own species. The one of each approach isn't good fishwatching, and is possibly cruel to the fish. We all struggle with that when we start with a decent number and have a gradual die off. Do we want to rebuild the group, or try something else?

A couple of years ago I bought a cory - Gastrodermis CW 123, which turned out to be a dull buy indeed. I lost a few after shipping, but the survivors survived. The last time I ordered from that seller, I saw he had them again at 6 for $49 cad, and so, I spent another $49. There was no pleasure in watching three do nothing, but nine of them are very active, enjoyable enough fish. It was a transformation.

If you have kinds of a tetra species, say a longfin, regular, albino mix in a group, then it works. Kinds, varieties and species are very different things though.

BlaiseinLondon · Jul 28, 2026

Are corys the exception to this please? I’ve seen lots of fish keepers saying that you can mix and match with those, something I’m interested in doing.

Naughts · Jul 28, 2026

BlaiseinLondon said:
Are corys the exception to this please? I’ve seen lots of fish keepers saying that you can mix and match with those, something I’m interested in doing.

Cories are a shoaling species, so the more of a species you have, the better they will do. All species of Corydoras can be housed together, giving reasonable consideration to relative sizes that could outcompete some for food.
It is best to have a few (say 5 if you can) of each species, but if you can‘t source that many, or if some die, a larger mixed group is preferable to too few overall.

FattyFish · Jul 28, 2026

BlaiseinLondon said:
Are corys the exception to this please? I’ve seen lots of fish keepers saying that you can mix and match with those, something I’m interested in doing.

Many, many years ago, I had a tank with just two bronze Cory. They seemed happy and lasted for years, but I do agree that these fish like to be in groups and are social with each other. I currently have six Panda Cory, and although they are often active at opposite ends of the tank, I often see three or four together. So definitely, the more the merrier!

GaryE · Jul 28, 2026

To me, an individual cardinal tetra consists of 10 fish. These are fish whose entire natural history is based on communications within their group, and even ten fish barely qualifies as anything for a species that lives in the hundreds surrounded by millions. It's the same for all open water, non territorial shoalers.

This is very inconvenient for us. You can keep individuals in mixed and matched groups of similar species. Should you? When I was a kid, I always had neons, glowlights and a couple of others in set ups like that. I regret it now though, as I suspect I was unintentionally cruel. The fish lived long, but when I look at how they behave in single species groups compared to how they acted in mix and match tanks, I want to kick myself. I lacked space and money, but above that, I lacked understanding of their needs.

My longest lived cory group fish was a solo aeneus that got well into her teens. I really liked her, as a pet, and looking back, if I had had a decent sized tank and an understanding of her social needs, I would have been a better keeper. If I had it to do all over again, I would have gotten her companions. The same holds true for all the little shoalers.

How many? Ten is an arbitrary number I don't trust. We had a member here, Byron, who was deeply concerned about humane fishkeeping. He researched and learned until the end of his life, and he advocated for large groups based on fish behaviour papers and studies he shared. They pointed to 10 as a baseline, but I doubt it stands. I would say six of a shoaler is adequate. Ten is much better, since adequate isn't great. More is merrier for them. Not necessarily for us, since fish are becoming expensive, and space for tanks already is.

I doubt many people have read this far, so....I was raised to believe we were above the animals and they were here for us. Now, it's clear we are animals like all the others, and we're part of a mesh of life. If I'm going to keep other animals in captivity, and I seem to have a need to do so, I owe it to them to try to give them a semblance of their world. In return, they give me a semblance of their ways of living to learn from. True, a cory shoal or a cardinal shoal can travel great distances in a day, and none of my tanks have great distances to offer. But I try.

Magnum Man · Jul 28, 2026

small fish -big fish… I suppose similar would happen with a larger fish ( like silver dollars ) though I‘ve never seen a home aquarium filled with 10 - 20 silver dollars ( this is the direction I’m leaning towards with my 250 gallon )… my smaller tank is an experiment, and though I started with 3-4 of each, by the time everything shook it’s self out, I have 4 singles, that act as if they are all the same species… with social interaction, and a hierarchy… I would suspect I would see similar in the 250 gallon, but I’ll be trying to get my shake down numbers to 4-5 of each… I’m not sure if I would learn anything different if there were 4-5 of each of 4-5 varieties, or 25 of the same variety???

Back in the fold · Jul 28, 2026

@GaryE You state that “ I doubt many people have read this far “ . Your post is not so long that it couldn’t be read in a few minutes . Are people really so short of time or have such weak attention spans that they can’t read through something so short ? I love wading through things to find the weird , the interesting and the funny . I used to like reading The National Enquirer and oddball magazines and pamphlets that you’d find in grocery stores and barber shops and and different places for that but now it’s the internet for my fix of the strange and bizarre and fascinating . Sad that people deprive themselves of what’s out there .