This article presents some of the basic concepts involved in managing water in tropical fish aquariums. The intent is not to address all problem situations and their solutions, but to inform the practitioner of the interactions between fish, water, and container and the complicated nature of successfully diagnosing water problems in the home aquarium. The important variables in water quality as they relate to fish health are presented. Potential problems from source water, inadequate biological filtration, and aquarium overcrowding are discussed.