How can you tell when a fish is dying of illness or old age?

How (if you can) tell if a fish is dying from illness or old age? For example if the fish you bought at the fish store was already old, but you didn’t know.

 

The fish you buy at the store are not usually full grown. You should find out the average adult size of a fish before you buy them. And sometimes you can find the normal growth rate too. at you can make a decent guess as to the age. If you adopted or purchased an adult size full grown fish, you are probobly out of luck sorry.
If one fish in the tank becomes ill and it is kind of old or full size for the species, and none of the other fish are sick, and there are no obviose symptoms of illness, old age happens.

 

As a fish ages their scales change, if your fish have dark edges on their scales, then it is an old fish. Young fish have bright scales, older fish have duller scales with black lines on the edges.

 

As a fish ages their scales change, if your fish have dark edges on their scales, then it is an old fish. Young fish have bright scales, older fish have duller scales with black lines on the edges.

I don't know any fish for which this is always accurate. Sorry, can't let that stand.

It depends very much on the species.

For example:
- A cardinal tetra will become paler coloured, with more white colour spreading from the stomach. Speaking of which, the stomach may become bigger or the fish gets thinner.
- An Apistogramma will also become thinner, the fins may grow all live and thus become oversized and start to curl or bend. Also these fish can become grey.

Other possible signs of old age: Blindness (cataract), less activity, slightly bend spines (like a hump or a caudal fin hanging down a bit limp), unusual slightly tilted swimming angles...

Generally one thing that shows if a problem is age related or genetic, is simply when bodily functions (buoyancy, digestion, responsiveness) stop working correctly without typical signs of illness: No loss of appetite, no loss of social status, no reclusiveness, no other outer signs.

 

I don't know any fish for which this is always accurate. Sorry, can't let that stand.

It depends very much on the species.

For example:
- A cardinal tetra will become paler coloured, with more white colour spreading from the stomach. Speaking of which, the stomach may become bigger or the fish gets thinner.
- An Apistogramma will also become thinner, the fins may grow all live and thus become oversized and start to curl or bend. Also these fish can become grey.

Other possible signs of old age: Blindness (cataract), less activity, slightly bend spines (like a hump or a caudal fin hanging down a bit limp), unusual slightly tilted swimming angles...

Generally one thing that shows if a problem is age related or genetic, is simply when bodily functions (buoyancy, digestion, responsiveness) stop working correctly without typical signs of illness: No loss of appetite, no loss of social status, no reclusiveness, no other outer signs.

It applies to all fish if you look closely at them.

 

I've had fish reach triple their natural age and I haven't seen this in them. Doesn't work out, sorry. Maybe you mean something else, but dark (edges on the) scales are not something you see in every group of fish, let alone every species, as a sign of old age.

Edit: Is it possible that you mixed that up with the age rings in fish scales? Those are visible under a microscope, but not with the naked eye and they don't necessarily have to be dark.