Is There Any Way To Find How Old Fish Are?

I am not exactly a fish person but I have taken care of quite a number of fish over the years in either small tanks or fishbowls. I have taken care of some Angelfish, various tetras, many guppies, a black molly, some goldfish and some other freshwater fish that I do not even recognize. Here in our country, pet fish can be bought out on the streets outside schools and churches at very low prices. The vendors unfortunately do not know what kind of fish they are.

Anyway, I have always wondered if there is any way to find out how old fish are. If they are born in your tank it would be easy to jot down their birthday but when you buy them outside, the vendors are of no help. I thought to ask here since it seems most of you are experts in fish. I hope someone can answer my question.

You can only tell how old some fish species are by their size, coloration, etc. which is different for each individual species. Once they reach maturity, it will be next to impossible to tell their age since most fish species look more or less the same on the outside once they are adults.

Most fish sold on the market are quite young. Usually they are less than half a year old.

The most accurate way (I believe this works for most species of fish) is by examining the otoliths. But this of course can only be done on dead fish. This is because otoliths are the inner ear bones of a fish. They are composed of calcium carbonate and protein and function as sound receptors. They also help the fish maintain balance and orientation.

There are 3 pairs of otoliths or ear bones in the inner ear of koi and goldfish (I am not sure about other fish species). The largest pair, the sagittae, are routinely used for determining the fish's age. What the biologist does is to count the number of visible rings under a microscope. White bands are formed during the spring and summer months, while darker bands are formed during winter. One white band and one dark band thus equals one year. Thus by counting the total number of bands (white and dark) under a microscope, we can accurately determine the age of a fish. But this would mean killing the poor fish to remove its ear bones!