Rabbit pellets are too high in calcium for guinea pigs and do not contain vitamin C. Guinea pigs are one of only a few animals that require a daily source of vitamin C in their diets because their bodies are not capable of producing this vitamin. A lack of vitamin C is the most common nutritional deficiency in guinea pigs. Guinea pig pellets are the best source of vitamin C; however, this vitamin deteriorates with time, so it is important to ensure that commercially prepared guinea pig foods are fresh and use a stabilized form of vitamin C. It is recommended to supplement guinea pig diets with fresh fruits and vegetables containing vitamin C or to provide vitamin C tablets designed for guinea pigs. You can add vitamin C supplements to their water, but it breaks down quickly with light exposure and may decrease water intake, so you cannot predict vitamin C intake from water consumption.
