Just to point out, chloramine is used in place of chlorine because it doesn't evaporate. If your city water uses it, aerating in a bucket won't work. You have to be sure a dechlorinator says it works on chloramine. As for filter changes, not sure why you would do that. It's where most of your good bacteria lives. Unless you are using activated carbon to remove something specific like medications, you never need to replace it. Even though the carbon in a stock filter bag looses its "activation" it still serves as a media for the bacteria to live on. And as was said, a full water test is the first thing you should do if you have a problem. I like to record my tests with dates for a while when starting a new tank(also noting any water changes and change in stock), and reference it when I test to make sure nothing is goofy. If I ever have something out of normal, I will add that to the records and keep recording until things get back to normal.