Betta's eye turning black?

[elenid]

I've just noticed this today, but it may have lasted longer since I didn't observe him that closely for the past few days. The black part of his left eye seems to have expanded. My betta's eyes are usually like the second photo (right eye).

He's swimming steadily and isn't lethargic, but doesn't seem to notice things with his left eye. I just did a 100% water change. Is this something I should watch and wait on, or do I need to do something else?

Tank: 1 gallon
Water changes: One 50% and one 100% change/week with water conditioner
Temperature: 72-76 degrees (closest it will get with Hydor heater in winter)
Food: 3 pellets, 6 days/week; occasional bloodworm
Filter, water testing: N/A
Betta: no history of illness, ~1 year old

[asukabetta]

I don't think there is anything wrong, one of my betta's has dark pigmintation of the eyes like so, perhaps it's your betta just gaining color there.

Can it see though the eye? Meaning if it's not blind or inflamed in anyway I don't think there is need to worry :)

[fleetfish]

He looks like he's a marble betta. Marbles tend to change colour rapidly, and this doesn't exclude eyes. It is a genetic and one very interesting pattern so don't worry if you end up with a completely different betta than what you had before. If is acting strangely, though, it would be good to seek treatment.

[majesticstorm]

Don't worry about it, it's perfectly fine. As long as he has a healthy appetite and active, don't worry too much about it. Both of my betta's eyes are pitch black, you can't even see the pupil, which I have no problem with since I have a soft spot for bettas like those XD

And fleetfish, I had no idea marble bettas' eyes could change colors too, that's very interesting.