Fish can become fat and they generally get a little more solid in the body and more rounded in the belly area. They normally store fat in the abdominal cavity where the organs are and not in the muscle tissue like mammals. The fat is not stored in the organs, but above them just under the spine. In extreme cases, small amounts of fat can be stored in the organs or muscle but that is very rare.
Fish and all animals are opportunistic feeders and will eat whenever food is available. Most fish can go for weeks without food and then gorge themselves when food becomes available. Their stomach can stretch and they can eat quite a lot of food compared to their body size.
