The red-tailed shark (Epalzeorhynchos bicolor) actually is extinct in its native range in Thailand and only survives in captivity. While some blame the aquarium hobby, which gets all of its fish from fish farms, habitat degradation is the more likely culprit. It is not possible for this fish to be pregnant.
The Shark Is Not Pregnant
Since the red-tailed shark reproduces with eggs, it is physically impossible for a red-tailed shark to actually become pregnant. However, female red-tailed sharks may swell up with eggs when breeding, though this is very rare in the home aquarium. In fact, the fish farms that provide this fish for the aquarium hobby usually spawn this fish in large outdoor ponds. A swelling red-tailed shark usually indicates that something has gone wrong, and the fish may require medical intervention.
All In One Basket
Rarely, a female red-tailed shark will decide it's breeding season in the home aquarium and produce eggs. If she doesn't spawn for some reason, the eggs within the body can cause health problems, a condition called egg-binding, or being egg-bound or egg-laden. This potentially can kill a red-tailed shark, and may require a veterinarian's attention to correct. To distinguish this condition from other medical issues, make sure the fish is female. Female red-tailed sharks have slightly wider bodies than males and slightly shorter fins. Additionally, this disorder does not cause bulging scales.
Herbivorous fish, like red-tailed sharks, can suffer from constipation. This happens most often when they do not receive enough fiber in their diet. In a constipated fish, you will see swelling of the abdomen without protruding scales. You also may see stringy feces and the fish could have trouble swimming. Treat this condition yourself by feeding high-fiber foods, like tinned peas or Daphnia, a crustacean you can purchase frozen or freeze-dried from the pet shop. Adding 1 teaspoon of Epsom salt per gallon of aquarium volume also can help alleviate the issue.
The Dreaded Dropsy
A swelling shark may may have dropsy. Technically, dropsy is a symptom of a disease, not a disease itself. In dropsy, fluid builds up inside the fish's abdomen. Organ failure, infection and "no apparent reason" all can cause dropsy. The major distinguishing feature of dropsy is that the scales protrude. Some describe this symptom as "pine cone scale." Unfortunately, for a smaller aquarium fish, like a red-tailed shark, treatment rarely works and euthanasia might be the best option to alleviate suffering.