When Is A Good Time To Clean A Fish Tank?

I am new to the whole aquarium world and just become the proud owner of 2 redcap goldfish. I have a whisper filterb and know that it needs to replaced evry month or as soon as there is little or trickles of water pouring out of it. The filter is seated on top so it is dipped and submerged into the water itself. That being said, my tank is a 1.5 gal tank, and I purchased it this past Wednesday and let the water sit over night with water conditioner in it. I got the fish Thusday and placed them in after putting some sort of biological aquarium supliment. First question, how soon would be a reasonable time to replace tank with fresh water, every week or every two weeks, I mean should I clean the water out as soon as this week Thursday? Secondly, after cleaning the dirty water out and placing fresh water into the tank and placing both the conditioner and supliment into the water, how soon after that do i introduce the fish into the their newly changed water. I was going to purchase a small carrying tank, the ones that baby turtles come in and let them swim in there until there while I clean their tank, and until the new water is ready and safe for them. But the question is how long would it take for that to be ready, and safe for them to swim in? The water doesn't have to sit overnight I hope. Please, and when I say please, I mean please help.