Does a fish tank need a filter?

Absolutely. In extreme cases you can filter and clean the water to an extent where you will kill off all of the good bacteria in the water that help the fish to thrive. You will then have to counter act the effects with over the counter nitrate drops or something similar. Alot of times tap water can be "too clean" and not contain all the vital Ph balances to keep fish healthy.

hi i have a fluval 304 external filter, filtering a 96 liter tank is it 2much ??

You can not filter a tank too much. The more filter media you have, the better your tank water will be. You can however create too much current for the species of fish you have in there. Most of the larger and better power filters have some method of turning them down to reduce flow.