After ruling out temperament, sensitivity to light, and the possibility of offspring, you need to consider stress as a reason your fish may be staying out of sight. Stress shows itself in a variety of ways, and there could be several causes of stress in your tank. The most apparent stressor is bullying from a more aggressive fish. Take note of any fish that are pushing others under a pile of rocks, into a corner, or behind a filter. If that's the case, form a backup plan with your exotic pet vet to keep your tank a happy place. Another stressor is illness. If you notice your fish is a little more standoffish than usual, look for signs of disease. Your fish, like humans, may prefer to be on their own if they’re injured or unwell. Keep a close eye on your fish if you suspect they’re sick or injured. Reach out to a veterinarian or experienced fish keeper if they don’t seem to be improving. You may need to quarantine the fish or treat the entire tank. Identifying the reason your fish is hiding will give you valuable insight and understanding of their needs and help you worry less about the health of your pet.