Planted tank with no filter ?

Why use filters in a planted tank at all?

The primary purpose of filtration in a planted aquarium is to break down organic wastes into simpler, less harmful substances using natural microbial processes.

The best known cycle is the conversion of toxic ammonia into less toxic nitrogen compounds by bacteria and other microbes. The faster waste compounds are digested by bacterial action, the more efficient the conversion process, the better for the animals and the tank environment.

Plants and substrate perform essentially the same functions-plants take up ammonia and nitrates, and the microbial community in the substrate breaks down organic waste. In this sense, a well-functioning, mature, planted tank can run smoothly without a filter as long as the uptake capacity of the plants exceeds the rate at which the animals produce waste.

However, filters are still useful in many ways...

Smoother start-up in new tanks

In a newly planted tank, filtration is particularly important because there is usually a lot of plant debris and VOCs from the transition stress. During transition stress, plants may melt or old growth may be replaced by new growth more suited to the new parameters. These can easily trigger algal blooms if organic waste is left undigested in the tank. The faster these organic compounds are broken down into simpler elements, the less of a trigger effect they have on algae. This is also why more frequent water changes are recommended in new setups for the first few weeks.

In mature, planted tanks with strong, stable plant growth and a mature microbial ecosystem, this is less of an issue as the microbial population can digest the organic waste produced. However, tanks can take many months to reach this stage.

Useful backup - systems can fail

Plants only take up ammonia/nitrates when they are growing well. Plants may not be growing well due to a variety of user errors - if they are not growing well, they are contributing nitrogenous waste rather than taking it up.

Having a filter adds an extra layer of stability to a tank in terms of processing organic waste products should the plants not be performing well at any given time. This backstop ensures that you do not lose your animals if your plants fail.

Most hobbyist planted tanks go through periods of flux and instability; a filter acts as a counterbalance to these periods.

Better Water Clarity

Filters maintain water clarity by trapping fine particles. In biologically mature tanks, a microbial biofilm binds fine suspended particles. Filters accelerate this process by introducing flow over a large surface area for bacterial colonization. If water clarity is poor - it may indicate that the bacterial cycling process in a tank is not mature. 

Most bacteria in a tank adhere to surfaces rather than float freely in the water column - so a filter provides a tremendous surface area for bacterial colonization.

Water movement & flow

For many tanks, a filter is used to provide water movement & flow. Water circulation in a planted aquarium distributes oxygen/carbon dioxide evenly throughout the aquarium and carries waste to the filter elements.

Given the importance of flow in a planted tank, it's important to have a pump for water movement if no filter is used.