It is not a steadfast rule that I only do a pwc every three to four weeks. If I feel that the fish are not as frisky or are just not looking right, I do a pwc; this is for my established tanks, which I do not test every day, but probably every three weeks or so or unless I think there is a reason I should sooner. Summertime, I do more because of the hot weather and I just think a nice fresh "bath" would be good for them more often.
Then I sometimes do one every two weeks.
Australia! Wow! Bet it gets pretty hot and sticky there? We had the prettiest trees this morning! We had fog last night and a temp. of around 4 degrees above zero and the trees were loaded with frost this morning and the sun shining on them. How beautiful! Our camera is on the blink or I would have gotten some awesome pictures!