I have an Aqua Clear 110 filter on my 75 gallon community tank and it does great. I wouldn't go any smaller then that. The first thing you need to do is read everything you can about the nitrogen cycle. Cycling a tank is the most important part of this hobby. It takes a while but will save you money and stress in the long run when your fish aren't all dieing. As stated above it really depends on how much money you want to invest and how much free time you have. A planted Discuss tank is awesome but you will have to be doing a lot of water changes and keep things clean. Most people don't start out with discuss but if you are patient and take your time and research a lot before hand you can run one without any problem.