My 2 males used to do that with the red Sunseed food which was supposed to be a natural foraging mix..... most of the pet shop brands of food actually are not good for your chin. I noticed that alot of pet store brands had corn as one of the primary ingredients (your chin should never be fed corn because their delicate stomachs cannot digest it properly which can lead to intestinal blockages and many other problems that will be extremely painful for him.)Try to switch to Mazuri orfor the UK Oxbow which is all pellets and it's much better for him. Always ween him onto new food over a 2 week period.

Oxbow Chinchilla Deluxe (2.2kg) - Paws247

what food do you feed? chinchillas are very fussy eaters and should ONLY be fed all in one pellets, a muesli diet is no good for a chinchilla and they will pick all the junk bits out and leave the rest, most rescue centers even have a clause in their contracts that the chin must not be fed muesli as its main diet.

charnwoods is one of the best chinchilla foods to feed.
other good foods are duggins, science select & beapher

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my chins on charlie chinchilla. he doesnt like pellets, and when i got him from the rescue he was already on the mixed food and they said it was fine, also thats what the chins at my college are on. ive tried pellets, tried to slowly introduce them and he doesnt eat um!

My chinnies are funny about pellets too, so i feed them a mix of Beaphar Care + and Science Selective. The thing with Charlie chinchilla is that your chinnies will just take what they want out of it (usually raisins, peas, corn) and leave the rest. Raisins are actually quite bad for your chinchillas because of all the sugar and i have heard that some chinchillas can get problems with their teeth because of this. My chins used to be on Charlie Chinchilla until i heard about all the bad things and i was told that Charlie Chinchilla was the best, Beaphar is really quite good because it has colour and its more of a biscuit than a pellet