I've never seen one laying on it's back normally. Mine tends to lean back too far when feeding, and fall onto his back.
He could have just found a comfy position, maybe he likes the current on his back?
__________________ 45g SW~percula clown, dusky jawfish, lubbock wrasse, antenna goby, hermits, snails
37g~angel, opaline gourami, australis rainbow, syno. euruptus, 6 kuhli loaches
10g~guppies, hoplo catfish, 3 kuhli loaches
5g~black CT betta |