I have 2 betta fish, a female and male, in separate (completely) tanks... There is nothing similar between the two. Hers runs warmer, his has live plants, she has larger dollar store stones, he has pebbles, etc. The list just goes on.
The one thing that is the same was a few days ago I found an algae thing in BOTH their tanks. It wasn't typical algae, it was a fluorescent green/blue color almost like a marker had leaked or something. It was on their filter pads (so removed the filters) and in his tank on his skull decorations.
2 days ago I noticed he was becoming blue around the gills. I found it odd, but he's blue, so no big deal.. now I have noticed SHE has it too! Same color even. She isn't blue, her body is neutral and her fins are red (blue streaked, though) but she isn't really blue.
It is just very odd to me... so to be on the safe side, is there any disease/illness (perhaps caused by an algae?) that can cause this? They seem very healthy and eat, swim normally, flair, aren't lethargic, gill function normal, and just their normal selves.