Why is my fish turning darker

A lot of time fish will change color as they mature and age out. This can be slight changes, up to a full on color change (such as going from white/black coloration to gold for example).

Other times they can change color due to them showing dominance in the fish (cichlids are known to do this as the most dominant fish will be a more unique color) or due to their gender (Auratus cichlids for example).

Sometimes this can be a HUGE change, although it normally happens through time. You might notice spots forming of different color, before a full transformation.

Otherwise if it seems to be a gray coloration or another type (let’s say a Neon Tetra turning white) than it actually might be a disease vs. normal color changing.

How can you tell if it’s normal vs a disease or something? Like does fin rot turn pieces darker or some other diseases?

Andy said:

How can you tell if it’s normal vs a disease or something? Like does fin rot turn pieces darker or some other diseases?

Most diseases will end up either fraying the fins, or if it’s a color change, will either change to white (like a fuzzy white), red/pink (due to sores and blood), or black (dependent of its it’s a fungal issue). In almost all cases though, the behavior of the fish will have changed (hides more, eats less, less active, etc). Although color is a good sign of a disease, there are other aspects you can notice by simply watching that fish for a small period of time that would be more detailed and informative on what they may have.

I would pay attention to the fish’s behaviour—is it eating the same amount, sleeping the same amount, still active and seems to be happy? Also, what species is your fish? That might help us figure out whether a natural color change would be expected.

I have had fish that have become a completely different color as they have gotten older. Some species do this by default while others will change their color due to stress, diet changes, or changes in the water. If the fish is not stressed, you haven't changed it's diet, and the water is balanced - it is likely down to genetics.

I have been surprised by some of the dramatic shifts in color I’ve seen with healthy fish as they age.

It's kinda cool to see a fish change colors either due to environmental reasons or due to maturing. I think my favorite is probably a fish changing due to mating reasons as the shift can be drastic.

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I wouldn't be too alarmed if your fish is changing colors as they get older, this is normal and happens as they get older. It is really awesome to see them changing colors as they get older. The only time I will worry is if they have other things happening as they get older such as their fins fraying or even a big change to their scales.