when you vaccuum the gravel, do it till 25% of water is out of tank, then fill. this way you kill two birds with one stone. to kill three, just vaccume when your water level starts dropping from evaporationm so you dont have to top off anything.
three birds..
edit: unless its in a humid room, where it doesnt evaporate. in that case, you can only kill two birds, every week or two.