Ido water changes about 1a mth.as need 1/3to1/2 max.alwayas use asiphon first on gravel?

is the rule of thumb 1in of fish per galloon alfish r simi to agressivebut they all get along well parrot,servin,fire belly cats ,shark.and 3med bottom fedders to keep bottom and sides clean/im gettin so frustrated about ready to sell all cant figureout water clariety prob.