You cannot get hung up on numbers. We are dealing with living creatures, not inanimate objects (though I sadly admit there are too many people keeping fish who do not seem to recognize this distinction. There are no numbers but there is factual evidence that too few do cause significant problems for shoaling fish. [Shoaling is the term I use rather than schooling, though neither is specifically defined anywhere so far as I know.] All species of tetra for example live in shoals/groups of hundreds. The need for a group of their own species is programmed into their genetic code or DNA. We know from studies by scientists that a group of ten will be much better adjusted than a group of five or three; this means they will be healthier, less stressed. The group of five will be under stress, leading to other issues like a weakened immune system, more aggressive, and be reticent to even feed. You canot say that 7 fish will be stressed but 8 will not. We only know that the more there are the better, and there is definitive evidence than ten will be better off than five.
Every aquarist has a responsibility to understand the needs of the fish species he/she maintains, and that means knowing that a shoaling species needs ten or more. That seems pretty simple to me. There are some species that do need more than 10. Reliable sources like Seriously Fish include such data in their profiles.
No one can be certain that any fish is "happy," simply because we cannot talk to them. And what exactly does "happy" mean to a fish to begin with? Another discussion, but I'll stay with the term for whatever meaning it may have. Experienced aquarists can detect whether or not a fish is likely to be happy, but often it is impossible to tell externally if something is or is not affecting the fish. And that is where we come to water parameters. Each species of freshwater fish has evolved over thousands of years to function in very specific water when it comes to GH, pH and temperature. Fish do not adapt to significant deviances from the habitat water. A study from 1985 examined the lifespan of cardinal tetras as it relates to the hardness of the water, and found that the harder the water, and the longer the fish was forced to live in it, the shorter the lifespan. No external signs at all. The fish just died. A necropsy of each showed calcium blockage of the kidneys due to the calcium in the hard water. Hard water fish like mollies must have this calcium in order for their physiology to function well.
Every aquarist has a responsibility to understand the needs of the fish species he/she maintains, and that means knowing that a shoaling species needs ten or more. That seems pretty simple to me. There are some species that do need more than 10. Reliable sources like Seriously Fish include such data in their profiles.
No one can be certain that any fish is "happy," simply because we cannot talk to them. And what exactly does "happy" mean to a fish to begin with? Another discussion, but I'll stay with the term for whatever meaning it may have. Experienced aquarists can detect whether or not a fish is likely to be happy, but often it is impossible to tell externally if something is or is not affecting the fish. And that is where we come to water parameters. Each species of freshwater fish has evolved over thousands of years to function in very specific water when it comes to GH, pH and temperature. Fish do not adapt to significant deviances from the habitat water. A study from 1985 examined the lifespan of cardinal tetras as it relates to the hardness of the water, and found that the harder the water, and the longer the fish was forced to live in it, the shorter the lifespan. No external signs at all. The fish just died. A necropsy of each showed calcium blockage of the kidneys due to the calcium in the hard water. Hard water fish like mollies must have this calcium in order for their physiology to function well.
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