Do you test your tanks then change them or just do the waterchanges every 3 days no tests? Maybe you should test your tanks every 3 days and if the ammonia or nitrite shows then do your waterchange accordingly; otherwise if filter is good and your ammonia/nitrite is 0 then leave it for the week and just do spot cleaning with a turkey baster.
It would only really harm them if you did major inconsistent % waterchanges in the space of a few days: ie: 30% one day, 50-90% next and subsequent days or all within space of a week. This method can throw out an established/cycled tank and cause it to cycle again, which invariably will stress your axies if you haven't realised it is cycling and drop back to doing weekly waterchanges.
Occasionally we may do 20-30% waterchanges daily if the ammonia and nitrite are above 0, to bring them back to safe levels.
(Message edited by kapo on March 22, 2007)