Yes, fish bowls need a filtration system. One of the best filters for fish bowls are sponge filters as they offer both mechanical and biological filtration without taking much space. They are small and are placed at the bottom of a fish bowl. One of the best sponge filters for fish bowls is hygger Aquarium Double Sponge Filter. It has two sponges that offer 3 stage filtration, is easy to set up and use, and is ultra-quiet.