I recently rescued a betta from a pet store and tried to get him healthy again. When I got him, he looked like he'd recently had fin rot - one of his pelvic fins was almost completely gone, and the rest of his fins were looking very shabby. But he's gotten a lot better, and he seems to be in perfect health. He eats pellets twice a day, swims in and out of his plants, and comes up to the side of his bowl whenever he sees me. His water gets changed twice a week (he's in a one-gallon bowl until I get paid and can get him a bigger one) and he isn't around any other fish - the other bettas are across the room, out of his sight. The bowl isn't filtered or heated - I live in Hawaii, so temperatures are usually in the high 70s-low 80s, but at night I have a lamp on right next to the bowl just in case.
Recently, though, he's developed black lines and patches on the front half of his body. They don't look like parasites - either the scales have turned black or the skin directly under them has. He hasn't been scratching himself or anything - he's completely fine except for the black lines. I know that females develop dark lines when they're ready to spawn, but I couldn't find any information on what these black areas would mean on a male. I'm not too concerned because he seems otherwise healthy, but I just want to make sure this isn't anything to worry about. I first noticed two or three scales darkening about a week ago, and now there are two vertical lines of black plus a few stray scales behind them that are getting dark.
Here are two pictures I managed to get - he was flaring at the camera so much that it was hard to get anything that wasn't too blurry. What I'm concerned about isn't the dark area around his gills (those have always been that color) but rather the dark lines just behind them.