How can fish swim in the dark without hitting objects?

Lateral line on this betta's body.Fish have what is known as a lateral line, running down each side of the body, from behind the eyes to the base of the tail. You may be able to spot this in some cases, as a lighter streak evident near the midline on the side of the fish’s body. It is a tube beneath the skin filled with a jelly-like substance, which is very sensitive to vibrations and connects directly to the brain.

The lateral line works rather like radar. As the fish swims, so it can detect waves in the water being reflected back off obstacles in its path such as rocks, or the approach of a likely predator, enabling it to take appropriate avoiding action. With this highly efficient system to guide them, so your fish will therefore avoid colliding with obstacles in their aquarium.Bubble-eyed goldfish

The only case where you may be advised to adjust the decor in the aquarium, and not include any rocks, is if you are intending to keep the so-called bubble-eyed goldfish, a variety distinguished by the large swellings under each eye. These fish can occasionally puncture their sacs on a rough, protruding piece of rock as they swim around their quarters.