Here is the relevant information about feeding pets on Pesach from my book Man and Beast:
Dear Rabbi Slifkin,
I want to ask you a question about my hamster over Passover. I do not know what I am going to do with her over Passover, as I have asked my Rabbi, he suggested that I ask you. He has said that I should give her away to a non-Jewish person that I know, because the food is not acceptable on Passover. What should I do? I don’t want to give her away for Passover, so could I not feed her only vegetables and sunflower seeds (her favorite) and throw away the other food? Even if I did give her away could I go over to the people that would be keeping her and play with my hamster? Please will you tell me the answer, as I am really desperate to know! Thank you very much and have a great Passover!
A person is not only prohibited from eating chametz (unleavened bread) on Passover, but also from owning it or deriving any benefit from it. Thus, one may not own pet food that contains chametz or feed one’s pet with it during Passover. One may not even allow one’s pet to eat chametz that is given to it by a non-Jew. However, a person is permitted to sell his pet to a non-Jew for the duration of Passover. The animal should be housed in the home of the non-Jew.
Jews of Ashkenazi extraction have a strict custom not to eat kitniyos – various legumes – on Passover. However, it is fully permitted to own such foods and to feed them to one’s animals. As long as they are stored away from any human food, kitniyos often make good pet food for Passover.
Many pet-owners are confused as to what to feed their pets on Passover, so we shall provide some practical guidance as to which foods are appropriate. Any change of diet should be done gradually, mixing in the Passover diet with the regular food several days before Passover. It is generally not advisable to give matzah to any animals, not even fish.
Dog and cat foods often contain chametz. However, several brands of food that are certified as being kosher for Passover are widely available. Note that the kashrus organizations that publish lists of products which are suitable for Passover often include a list of commercially available pet foods which are suitable. It is not necessary to buy a new food bowl for Passover, but the bowl should be thoroughly cleaned.
Commercial food mixtures for small animals such as rabbits, hamsters, and so on, contain problematic grains. It is best to feed them fresh vegetables, alfalfa, sunflower seeds, and dry corn. Commercial alfalfa pellets can also be purchased.
Birdseed likewise usually contains problematic grains. It is best to feed them with sprays of millet. Birds of the parrot family should be given sunflower seeds, as well as fruits and vegetables.
Most fish-food contains chametz. A simple solution is freeze-dried brineshrimp, bloodworms or tubifex worms, which are available at all pet stores. If one is going on vacation, note that vacation “blocks” for feeding fish usually contain chametz. However, a healthy aquarium, with a mixture of fish and plants, can sustain itself for the duration of Passover without any food at all.
 Shulchan Aruch, Orach Chaim 448:7.
 Mishnah Berurah 448:33 and Aruch Hashulchan 448:12-13.