Turtles have a bony shell in addition to the scales that are part of their skin. This differentiates them from other reptiles, because not only do they shed their scales for larger ones as they grow, but terrestrial species also produce wider shields under the old ones to grow their shell. A thin layer of epithelium is responsible for covering the unions of all these shields.
The scutes of land tortoises remain on their shell forever regardless of wear. Meanwhile, aquatic species follow a somewhat different process. The outer layers of their scutes are shed each year to remove algae and other debris from their shell and thus allow them to swim lighter.
There are certain signs that your terrapin is starting to shed its shell. One of the most curious is that they will seek to sunbathe, since this way they absorb a greater amount of UVB rays, which help them keep their shell healthy for as long as possible. Regarding its behavior, it is likely that your pet will begin to rub its shields against the walls or decorative elements of its aquarium.
Changes in the color and texture of the shells, more or less evident depending on the species, can be another sign that your turtle is about to shed its shell. Normally it becomes brighter than before.
Both sea turtles and some freshwater turtles shed the scutes of their shells and, for this, optimal conditions of humidity, feeding, and levels of sun exposure must be met. One way you can help your pet during this process is to provide a diet rich in vegetables, which will provide them with the necessary vitamins so that they shed their shell quickly. Your tortoise also needs vitamin D, so try to let it sunbathe a number of hours a day, without exposing it to excessive heat.
Water quality is another important aspect. To avoid contracting diseases, change the aquarium water frequently and use a powerful filter to combat chemical and biological agents. Remove any sharp objects from its environment that could injure it, and replace them with clean rocks that it can safely rub against. If you see spots appearing on the shell or it is getting too dark, you can scrub the shell with a soft toothbrush to remove the dirt, apply an antiseptic, and allow it to dry before returning your turtle to the water. In this way you will avoid the formation of fungi.
Once the process has started, the shell of the turtle will come off little by little. The scutes should appear translucent, and while it's normal for your turtle to try to eat them, it's best to remove them from the bottom of the tank as soon as you see them.