The main reason for a toilet tank not filling or filling slowly is due to the toilet fill valve. The most common reasons for the fill valve to slow down or no longer fill the tank after the flush are debris issues and the length of time a fill valve has been in use....
Re: Fluidmaster slow leak/hissing noise - how to repair? Author: redwood (CT) If you pick up the float does it stop? If it does the toilet probably has a leaking flapper and the valve is just refilling to keep the proper tank level. If it doesn't stop replace the fluidmaster valve...
The volume of water in the flush tank may be too small. When the toilet tank refills, notice where the water level is when the fill-up stops. The level of water in the toilet tank should be within 1/4" of the "toilet tank fill line" marked inside the toilet tank. ...
Other suggestions include using dual-flush or compost toilets, installing low-flow showerheads, and replacing the lid of toilet tank with a sink-lid.MADRENCARRIEE: The Environmental Magazine