what kind of dechlorinator are you using? some brands will not detoxify chloramine, and your city's water supply may have switched over from chlorine (which most dechlors will handle), to chloramine recently. Prime is the one commonly found dechlor that works with chloramine treated tap water.
are the fish showing red gills at all? have you done an ammonia test?
you posted your nitrate result as zero. unless your tank is chock full of tons of plants, there should be nitrates showing.
have you cleaned your filter lately? how did you do it?
call your water supply and ask if there has been anything different going on with how they treat the water.
find out if anyone in your house used your water change bucket for any other purpose lately.
right now, drop your water level down so that your filters splash the water surface more - this will create better gas exchange and allow more oxygen in the water.
until you figure out the problem, don't do water changes with your tap water, in case there is an issue with it.
adding carbon to the filter is a good idea right now, to pull out any pollutants that might be in the tank water.