Are you treating the tap water before you add it to the tank?
before I made a water conditioner, I would fill a 5 gallon bucket with water and put a airstone in it for a day
and also added the water conditioner this way any extra dissolved gasses like ozone can be released.
And does your tank have a heater. if so what temp do you have it set to.

changing the carbon is good but don't trow out the filter bag it holds a lot of helpful bacteria
Just rinse it under cold water and rub off any extra debris.