About a week ago I bought a Betta fish and I'm wondering how often I need to change the water? Do I do 50% water changes weekly or more than that? It's a 2 gal tank with an air pump and heater but no filter. Also, my Betta fish won't eat, however I've read that's common when they're new because they might be stressed. But I've been feeding him pellets from PetSmart made for Betta's twice a day (I also have freeze dried bloodworms but he won't eat those either) and they're starting to pile up in the gravel and it just looks gross :/ so when can I clean the gravel without messing up the chemical balance or stressing him out further?