Toilet tank looses water over time
As long as there is no water leaking from the tank to bowl connection, you are OK. You would know if it was leaking here, because you would have water dripping out onto the floor.
Verify that the water level in your tank is below the top of your new flush valve's overflow tube. If the water level comes to the top, it will overflow into the toilet bowl. This can be easily fixed with an adjustment to the fill valve.
If the water is not overflowing into this tube, then the only way you can lose water is through the flapper. You said you changed the flush valve, which would have come with a new flapper attached to it, but maybe buy a new flapper and see what happens??? Read More At : Aqua-Saver 12" Two-Piece Pressure-Assist Toilet - 091...