The idea is great, some local Aquarium society keepers have set ups like this.
A couple thoughts are that the water for the tank should still be filtered, though I know some of these guys do not, They just adjust the amount of water exchange to account for feeding and waste.
What I have seen is that without cleaning the bottom of the tank, fluffy /mulm like stuff builds up. Also to keep up with good water circulation to help move stuff out through filtration.
The other thing which happens is that this water coming from the city treatment facilities may end up with very potent chemicals in it from maintenance, repairs and general flushing of lines to kill off bacteria to create "safe" drinking water.
Just be aware.
I would like to do this with pretreated water from a storage container and dosing pump for Prime. Personally for out water I need to add calcium and magnesium on a regular basis to keep the water stable. Not there yet but it sounds good!