Matured planted tanks with a low bio-load vs plant ratio may not have great need for biological filtration. However, filters are useful for capturing debris and also as a backup in the event that plant growth falters (and there is increased organic waste from the plant themselves).
6-10X Turnover flowrate
For many planted tanks, the filter functions as the main source of water flow. In a planted tank a flow of around ~10X turnover of tank volume is my recommended 'target' flow rate to achieve optimal circulation of O2/CO2 - and for delivering nutrients to plants. Spending on a filter with good flow negates the need for extra pumps in the tank. Less clutter in the tank means cleaner presentation and more room for aquascaping. Of course, the filter should operate with minimal noise, and the better ones are able to do with ease. In other words, strong AND silent flow is achievable, widely available and should not be compromised.
A filter has to be cleaned regularly to be effective. There is often the misconception that a filter is set up once and left to run 'forever'. This contributes to algae problems, especially in heavily planted tanks. To make the chore of regular maintenance less onerous, easy accessibility is key. A priming function enables the starting of the filter without the need to manually start the downward flow of water in the filter intake pipes. (i.e. sucking on the water hoses). It enables far easier maintenance work and setup. Most modern filters have this by default.
Easy accessibility is the best way to encourage regular maintenance and tank cleanliness. And nothing helps this more than a pre-filter compartment that can be cleaned without needing to take the main filter apart. Planted tanks produce a lot of debris - that should be removed regularly. Having an easy access system improve tank maintenance a lot. We find this an essential feature that folks overlook. Many filter models do not come with this - the two models that we use that do are the Oase Biomaster series and the larger filters from Aquael.
Less equipment in the tank maximizes space for aquascaping. Being in the center of flow ensures that heated water is distributed evenly throughout the tank. Less clutter in the tank means cleaner presentation and more room for aquascaping.
Quick release valves
This enables the filter to be serviced without fumbling with the water hoses. Quick release shut-off valves allows one to remove the filter from the water pipes that run into the tank instantly without mess/dripping water. It is easy to install your own valves if the filter comes without these.
Filters last a very long time if they are built of quality materials. Poor quality models have higher risk of leaking and breakdown.
Why it all matters
Having good flow in the tank that sweeps debris towards filter intake keeps the tank cleaner for longer. Overall tank cleanliness is one of the fundamentals for an algae free tank. In new setups and through periods of flux, planted tanks can create a lot of organic debris even with no livestock. A filter is thus useful as a collection point for free floating organic detritus. As a planted tank matures, it creates a mini ecosystem that digests livestock waste well - matured planted tanks can often run well without additional filtration if stocking levels are light. However, having a filter as a backup is a still good idea.
For a detailed review on Oase biomaster 300, click here.