The short answer
For a normal stocked tank, a filter is essentially required. It houses the beneficial bacteria that turn toxic fish waste into far safer nitrate, and it keeps water moving and oxygenated. You can technically go filterless in very specific, expert-managed setups, but for almost everyone — and every beginner — a filter is a must.
Why a filter matters so much
A filter does two jobs at once:
- Biological filtration — its media is home to the bacteria that process ammonia and nitrite, the invisible toxins from fish waste. This is the heart of the nitrogen cycle.
- Water movement — it circulates and oxygenates the water and stops stagnant, low-oxygen pockets forming.
Without that bacterial colony, ammonia climbs fast and quickly becomes lethal. A filter is what keeps a stocked tank stable between water changes.
The rare exceptions
You’ll see filterless tanks — heavily planted “Walstad-style” bowls, or ponds — but these work only because dense plants and a very light stocking level absorb waste, and they demand experience to balance. Even then, most keepers add gentle filtration for insurance. It’s not a shortcut for beginners.
The easy way
The reliable path is a filter matched to your tank. A gentle sponge or internal filter suits small tanks and shrimp, while a hang-on-back or canister handles larger community tanks. Browse aquarium filters to compare, and see do I need a filter for my aquarium and how to cycle an aquarium to set yours up right.