This is especially true if you only add male fish to your tank. Female fish, however, could be pregnant when they are purchased, and depending on the species, may produce fry for several weeks or months after their last contact with a male. To be on the safe side, only opt for male fish in your tank.
What does a fish look like when pregnant?
Does your fish have a red or black spot? Sometimes, when a female fish is pregnant, she will develop a “gravid spot” on the abdomen. This is typically black or red in color -and grows more pronounced over the course of the pregnancy.