A filter serves to break down organic waste, and also to provide water movement and flow. Of the various types of filters on the market, we would not recommend the following designs:
Hang-on filters for aquariums function well enough for many fish-only tanks. However, many of them produce an unfavourable flow pattern that changes in accordance with variation in a tank's water level. Many of them are designed with a downward pointing rather than horizontal outflow direction, which we find less ideal. We find them usable on low tech tanks, but hard to manage on CO2 injected tanks, especially ones that depend on flow to distribute CO2 mist.
Sponge filters powered by bubble lifts
For sponge filters in a CO2 injected planted tank, the bubble lifts off gases quite a bit of CO2. Also, unless there is supplementary flow through additional pumps, we find that most sponge filters don't give optimal flow patterns in a tank.
Such filters also take up valuable space in the tank and are hard to hide as well. They are usable in low tech non CO2 injected tanks as a budget option, but in larger tanks, there are always more optimal choices.