Filters are not completely necessary if you have live plants. I would still add a powerhead with a sponge prefilter to keep water moving and help collect particles from the water. Later on, needing a filter will depend on how many fish you have. If you don't have many fish, a filter is not necessary. Generally in planted aquariums you do not want to have biological filter media, or carbon, so really all you need is something to collect particles from the water and move water around. A powerhead with a sponge on it does that. If you have a lot of fish though, a filter is a good idea.
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Melissa
Aquariphiles.com - where I blog about aquarium stuff