Egg hatching problems

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None of these have died and they all are still spawning regularly. What is the 'normal' lifespan of a corydoras ?
"maximum range is 268ppm" maximum for what.? Do they all die off at 269? Aquarium cories are so far removed from wild that the wild parameters are all but meaningless.

The last sentence is somewhat misleading if taken at face value, so I will try to explain a bit so it is better understood.

Each species of freshwater fish has evolved to function best in a very specific environment, and here the word means everything from water parameters to habitat to numbers to other fish. The range for parameters is very narrow depending upon the species. Some have no ability to adapt, others can adapt to some degree; for the latter, it is not really a matter of generations of tank raising but more a matter of the physiology of the fish species. As soon as the fish is forced outside its preferred range, it begins having trouble functioning. The fish is slowly weakened; stress occurs, and if this becomes acute it wears down the immune system. Most fish in such inappropriate conditions die from issues they were no longer able to fight off because of the weakening. A shorter lifespan is therefore generally the result. Necropsy at death by a microbiologist may determine the specific issue that caused the immediate death. But in many cases, the lingering weakening effect just wears the fish out faster and it dies.

We are here dealing with the physiology of the fish, the intricate relationship of all the biological processes that keep the fish alive, feed its tissues, maintain its immune system, etc. Delve into fish physiology some time and you will see how incredibly complex the life of a fish is, much more closely tied to its aquatic environment than any terrestrial animal is to their land environment.

Corydoras have a longer lifespan than most realize. Ian Fuller documented one Corydoras that lived into its 40 year. No one is saying they will all live this long, but science is certain that unless they are maintained in the proper environment they will never achieve close to this.

I frequently write that ranges for GH, pH, and temperature are arbitrary and can only be a general guide. Inexperienced aquarists are always asking for limits on what they can force upon their fish, with in most cases well-meaning intent, but in all cases it is a false security. The habitat parameters for the species is the only way to know that the fish is in the best environment, because we know it has been designed to live in that environment and no other. There is no significant fluctuation in parameters in the natural habitat of most species we keep. The same species may have evolved to function well in two or more somewhat different environments with respect to parameters, (the Pristella maxillaris tetra is one such example, Corydoras aeneus is another) but this is not universal across all species. And it is not carte blanche for any extreme.

The slow effect of weakening is not visible externally until the fish dies (or succumbs to some other issue that it should have been able to deal with under better conditions). Like most all animals, fish will try to make the best of whatever environment we force upon them. That is because of every animals' incredible will to survive no matter the situation. But the fish swimming around, eating and spawning does not mean it is thriving; it is surviving. And knowingly keeping any species in parameters or conditions contrary to what they are designed for is just plain inhumane.
 

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