Why does my girl fighter get red marks on her gills?
Bettas develop two red bars on the gills when they display to one another/their own reflection. If she’s got these marks she’s clearly either showing off or telling the male to back off. Are you breeding? (The ONLY time a male and female should be together for a max of 48 hours or so) in which case she should really be displaying vertical bars down her body and be full of eggs after conditioning and the male should have built a bubble nest. Males and female bettas, shouldn’t as a rule be kept together, one or the other will attack the other at some point, often fatally.
