A filter is definitely optional. I think a lot of people overlook the benefits of unfiltered tanks. Most of mine are and they are 15-20 gallon tanks with full stock. If you are planted well then it is fine. You should see no ammonia or nitrite at all. The benefits of no filter is a much more natural system that very often has a lack of nitrate production. Its not uncommon for nitrate to go down between water changes in some of my tanks.