Hmm.
What's the lighting in the tank? Given there's one plant the fish probably doesn't feel secure. If I had to guess I would say the fish spooked, hit its back on the lid and got a lower spine dislocation.
Any pests (kids, dogs, cats, wives) in the house that could cause the fish to spook as they run past, tap the tank, or cause loud noises? How is the fish swimming? How often is it fed? Are you feeding the feeders? Malnutrition could possibly be a factor in this.
What's the lighting in the tank? Given there's one plant the fish probably doesn't feel secure. If I had to guess I would say the fish spooked, hit its back on the lid and got a lower spine dislocation.
Any pests (kids, dogs, cats, wives) in the house that could cause the fish to spook as they run past, tap the tank, or cause loud noises? How is the fish swimming? How often is it fed? Are you feeding the feeders? Malnutrition could possibly be a factor in this.