It is only with years of experience that an aquarist can judge the age of many fish species. Zebra Danios are fully grow at 4 to 6 months old and only live about 3 years or so, so it is a bit difficult to tell how old they are.
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By its general size and appearance. Most Tetras are not long lived fish and depending upon species, live for 3 to 5 years. I would advise buyers to always buy from a reputable breeder if possible. If...
Its really a jugment call the longer you have fish in a tank, or choose to breed fish the more knowledge you will aquire in being able to determine to some degree how old a fish, or school of fish...
Size is a good indicator of age; basically the bigger the older. If you know the exact species and their growth rates and sizes, you might be able to tell their age to within a month or two. For some...