What can people feed their fish over Pesach?
Is it possible to feed them regular flakes, even if they contain chametz, since it is inedible anyway?
R' Cohen in Dose of Halacha quotes R' Ben-zion Abba Saul who does allow you to use flakes, though writes that others advise you to buy worms, etc. or something that is chametz free.
Rambam (Chametz Umatza 4:8) and the Shulchan Aruch (OC 442:4) write that one does not need to dispose of a mixture containing chametz that is totally inedible (by humans). Thus, R’ Ben Zion Abba Shaul (Ohr Lezion 3:8:5) allows one to feed one’s fish with regular fish food on Pesach even though it contains a little chametz, as there is no way that such food can be considered edible.
Nonetheless, R’ Avrohom Blumenkrantz (Chasdei Avrohom 21) recommends one to buy kosher l’pesach food, such as dried worms, etc.
I am linking here to the Star K guide http://www.star-k.org/2015PetFoodList.pdf. That may be a help.
I will quote what they say about fish food.
"Fish food and vacation blocks often contain chometz. Tetra Tropical Slow Release Gel Feeders (Tetra Weekend 5 days, and Tetra Vacation 14 days) are chometz free. Goldfish and Tropical fish can be given tubular worms, frozen brine shrimp, and freeze dried worms (if they do not contain fillers)."