Current radio telescopes have imaged the HI in and around a few hundred galaxies in the nearby universe, but to reach out into the universe to the time when the decline in star formation started will require new facilities. An essential ingredient in the process of galaxy formation and evolution is the balance between the acquisition of gas through merging and accretion and the depletion of gas through stripping and outflows induced by star formation and in some cases active galactic nuclei (AGN). This balance will depend on the environment. The life long gas supply and regulation of this balance will determine what kind of galaxy emerges in the end. The strong decline in star formation density in the universe over the last 7-8 Gyr indicates strong evolution. To be able to understand this evolution it is necessary to assess the balance between the acquisition and depletion of gas in many individual galaxies and many different environments over this period of cosmic time.In less than two years the Westerbork Synthesis Radio Telescope (WSRT) will be upgraded with a wide field capability using the novel technology of phased array feeds (PAFs) developed in the Netherlands. This system, named APERTIF, will make it possible to survey large areas of sky rapidly to depths covering the last 3 Gyr of cosmic time.This proposal requests funding for harvesting the huge potential of the APERTIF system. It focuses on analyzing the data of a blind, medium deep survey of HI out to a distance of 1250 Mpc (a redshift of z = 0.25) covering an area of ~500 deg2 (or a volume of over 30 million Mpc3) to explore the balance between gas acquisition and gas removal processes in galaxies in different environments. This balance is fundamental to the way galaxies evolve. Primary goal is to obtain a full inventory of this balance and show which processes dominate in which environments.