Does filtration matter less in a planted tank?
3 PURPOSES OF FILTRATION
breakdown of organic waste
The main purpose of filtration in a planted tank is to break down of organic waste into simpler, less harmful substances by making use of natural microbial processes.
The most well known cycle is the conversion of toxic ammonia into less toxic nitrogenous compounds by bacteria and other microbes. The faster waste compounds are digested by bacteria action, the more efficient the conversion process, the better for livestock and the tank environment. One hint that the bacteria processes in a tank are not matured is when tanks smell bad.
In a planted tank filtration is important in new tanks as there is usually a lot of plant debris and volatile organic compounds produced when a newly planted tank settles in. Plants may melt during transition stress, or old growth is replaced by new growth more suited to new parameters. These easily trigger algae 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 is recommended in new setups for the first few weeks. In matured planted tanks, with strong stable plant growth and a matured microbial ecosystem, this is less of an issue as the microbial population can digest the organic waste produced. However, it can take tanks many months to reach that stage.
These microbes are delicate and heavy usage of harsh chemicals (like algicides) will affect them. This is why many aquarists get caught up in the never ending loop of killing algae - having new algae - looking for new cures, whist never having the tank in a sustained period of being algae free. Prevention is better than cure - if you dose algicides, spot dose them in problem areas early, rather than nuke the whole tank.
As a whole, it is good to have good filtration as a backup. It adds an additional layer of stability to a tank in terms of processing organic waste products. Most hobbyist planted tanks do go through periods of flux and instability, so in my view having good filtration is essential, with good filtration/flow being highly beneficial to keeping a tank clean and free from algae.
Filters maintain water clarity by capturing fine particles. In biologically matured tanks, microbial bio-film binds fine suspended particles together. Filters speed up this process by introducing flow over a large surface area for bacteria colonization. If water clarity is poor - it can hint that the bacteria cycling process in a tank is not matured. Instant cycling products have improved over the years, and many on the market work well to shorten the amount of time it takes for filters to mature.
Most bacteria in the tank adhere to surfaces rather than free float in the water column - so a filter provides tremendous surface area for bacteria colonisation.
water movement & flow
Provide water movement & flow. Water circulation in a planted tank distributes oxygen/carbon dioxide evenly throughout the tank and brings waste to filter elements.
Often it may be the only form of water movement in a tank if no supplementary pumps are used. Giving the importance of flow in a planted tank, it's important to choose a filter design that encourages maintenance/clogs less easily. Clogged filters can have their flow rates slow down significantly.