Aquia III water tank level dropping

Just installed a brand new Aquia III and the water level in the tank is slowly decreasing (about 1/4 inch an hour). I can see water trickling very slowly into the bowl from under the rim. I cleaned the gasket on the flush valve and inspected the valve seat. Both look fine. The gasket on the valve is clear vs black as described in the owners manual, but it looks to be in perfect shape. I've also pulled the tank and checked to ensure the flush valve mount nut is tight - it is. Would appreciate any other thoughts/advice. Thanks!

If your leaking from the tank into the bowl, then you have checked exactly what you need to.
The gasket is clear and not black on the one's i have as well.
If it is still leaking than i suggest you either go back to where you got it and get a new flush valve or call Toto direct 1-888-295-8134 (press 4) and tell them and ask for a THU224.
They will send it out to you direct.

Thanks very much for the reply and the phone number. I called them and they are sending a new flush valve gasket - perhaps that will solve the problem, but I suspect I'll be calling them back next week for the entire valve assembly! How does that bottom base of the valve assembly (the part held on by the large nut on the bottom of the tank) seal? Is there also a gasket between that part and the tank?

Yes there is a black bevelled gasket.
It doesn't show it as a replacement part though, you would need to get the entire assembly.
I don't think that gasket would be leaking though.