Betta fish are the best for your situation....They also called (Japanese Fighting Fish).....
These fish are good because they can be kept in a tank as small as a soda can. Plus there is no filter needed to clean out the tank....If the water gets dirty, the fish will not die as quickly as other fish types. Also no need to worry about feeding these fish, they can go for days without eating if you ended up busy or go on a weekend gettaway... These fish come in all kinds of colors.....Red - Red/Blue- Orange/Red - Purple- Greenish Blue etc....The only thing is that these fish are usually but not always loners...I had my Betta fish in with gold fish and my black moor fish and the Betta bit some of the other fish in the tank
You can get Betta fish at some Walmart department stores or any pet store, This is a good fish to have if you want to train your children who as young as 2 years old on how to care for a fish before the children move on to having the more exspensive, more need for care types of fish...You cant really mess up with these fish...Bettas are great starter fish and are very cheap and easy to care for.