with a 16g and a cupple fish you could get by with a good canister like a fluval <sp? if you start getting 4 or more fish youd want to invest in a hang on skimmer I tell people to by a used super skimmer untill you decide if you want to advance to a bigger tank (most people get a bigger tank within a year) then you can save and get a good sump/ refugium/ skimmer system. all in time I guess the best thing is to not by just enough filtration then regret it. I have a whole garage dedicated to that. theres my two pennys