Yes: for a normal home guppy aquarium, use a filter and keep it running day and night. Its most important job is supporting the microorganisms that process fish waste, not simply catching visible dirt. A mature sponge filter can work very well, particularly where young fry are present.
Clear water can still contain harmful ammonia or nitrite. Looking through the glass and deciding that everything seems clean isn't a water-quality test.
What the filter actually does
Mechanical filtration traps particles such as food and plant debris. Biological filtration provides surfaces where useful microorganisms live, supplied with flowing water, oxygen and waste products.
As the aquarium matures, those communities convert ammonia into nitrite and then nitrate. Ammonia and nitrite should remain undetectable in an established, properly stocked tank. Nitrate still needs managing through maintenance, sensible feeding and, where present, plant growth.
A filter therefore helps you maintain safe water; it doesn't remove the need for water changes. Captured waste also needs removing when you clean the filter appropriately.
Is a sponge filter enough?
A suitably sized, mature sponge filter can be a good choice for guppies. Air-driven models offer gentle water movement and avoid the exposed suction intake that can endanger tiny fry.
The air pump, tubing and air supply must be appropriate for the filter. A sponge sitting in a tank without water moving through it isn't doing the same job.
For a display aquarium, an internal, hang-on-back or external filter can also work. Match the equipment to the aquarium and stocking, and protect intakes if babies are likely to appear.
Avoid turning the aquarium into a current test
Fancy guppies with large tails should not have to fight the filter outlet all day. Aim for useful circulation and surface movement, with quieter areas available.
If the fish are continually being pushed about, adjust or redirect the outlet according to the filter's instructions. A spray bar or suitable baffle may help. Don't choke off flow in a way the manufacturer advises against.
A powerful filter does not make a small aquarium suitable for unlimited fish. Swimming space and stocking still matter.
A new filter is not a mature filter
Installing equipment is the beginning of preparation. A new filter has not instantly developed the capacity to handle fish waste.
Cycle the aquarium without fish, using an appropriate ammonia source and testing to confirm that the system processes it. There is no universal number of days after which a tank becomes ready. A bowl of fresh water may test at zero ammonia because nothing has produced any yet.
After adding fish, continue testing and increase stocking cautiously. An established filter can still be overwhelmed by a sudden rise in waste.
How should you clean it?
When flow drops or accumulated debris needs removing, gently rinse reusable biological media in removed aquarium water or suitably dechlorinated water. Avoid soap, boiling water and replacing all mature media at once.
Keep media wet during maintenance, minimise disruption and follow the equipment instructions. Some chemical media need periodic replacement; that is different from throwing away the entire biological filter on a calendar schedule.
Turn electrical equipment off safely before opening or removing it. Before walking away, check that everything has restarted and water is flowing normally.
Can plants replace the filter?
Specialist planted, low-stock systems can be managed differently. I wouldn't make one the default recommendation for a new guppy keeper, particularly with a breeding population.
Plants are useful, but “planted” doesn't tell you whether the aquarium can handle its waste load. Start with reliable filtration and learn your water test results.
If you want the setup explained alongside feeding, maintenance and breeding, Guppies brings those decisions together. The aim is an aquarium you understand, not a cupboard full of equipment you were told to buy.
