Does anyone have any ideas what could be causing this - or if it is actually a problem? This is a female protomelas stevenii who otherwise appears to be totally healthy. She eats well, and is active in a 180l malawi setup (sharing with 5 cynotilapia afra, 1 nimbochromis livingstonii, 1 Sciaenochromis ahli and 2 synodontis nyassae). They're on a diet of spirulina flakes - with tubifex worms or brine shrimp every 2 or 3 days. No-one else shows these symptoms - and apart from her stomach, this fish shows no signs of stress or weakness (never even a torn fin).