A good diet for guinea pigs is important to reduce teeth, gut and bladder problems.
Hay: 80% of the diet should be hay and/or grass. Timothy hay is ideal, it is low in calcium good for the guts and coarse for the teeth. Avoid too much new growth grass or clover, it can cause bloat.
Pellets: Approximately 1 egg cup full of pellets a day per guinea pig. Pellets must be labelled for guinea pigs, as they will contain a vitamin C supplement. Rabbit pellets do not have a vitamin C supplement. Pellets are better than muesli mix, muesli mix can cause selective feeding.
Fresh Vegetables and Fruit: Vegetables and fruit should be used as a treat. Avoid too much 'watery' vegetables (e.g. iceberg lettuce). Avoid feeding excessive amounts of dark green leafy vegetation (e.g. spinach, kale, greens) as it tends to be high in calcium. This can increase the chances of bladder stones.