This invention provides a filtration system for home aquaria that uses a closed-circuit pathway having a surface that contacts a flow of water moving between a tank inlet and tank outlet. The surface allows for algal growth thereon, and for transmission of light thereonto so as to provide photosynthetic energy to the algae. The algae acts to scrub contaminants (nitrogenous compounds, for example) as part of its growth cycle, and thereby purify the water. Additional components, such as injected air and filter media can also be provided to the water circuit to enhance algal growth, oxygenate the water and remove particulates. In an illustrative embodiment, the surface is transparent or translucent and a light that transmits appropriate wavelengths (for example, full-visible spectrum) is placed in proximity to the surface.