Fluidized bed filters are one of the best choices for planted aquariums. The filter will not drive off the valuable CO2, yet it is efficient and very low maintenance. Although the bacterial bed takes longer to establish, once it is set up it does not have to be disturbed when performing maintenance, as other filtration systems must.
All Rainbow filters are solid, and this one is no exception. It holds a large volume of media, is easy to fill, and I like the flatter design which is well suited for tight spaces. Like all Rainbow products, the design makes it easily connected in tandem with other components. It can be concealed below the tank, or hung off the back like a power filter.
Merlin offers the option of changing to other types of filter media, a real plus if you aren't sure about taking the leap to a fluidized bed. It does have a flow control valve, which some models lack. However, it only has inlets at the top, which precludes the option of plumbing it in tandem with anything else.
Available in several models are compact, simple, self-cleaning, quiet and easy to use. They are primarily designed for commercial use, however the sump model is within the price range of hobbyists.