I installed the above named fluidmaster 404lg and the water slowly disappears from the tank. Also the water pressure in the shower has diminished. Do you have any ideas?

Not related to each other. First the Fuildmaster, here is an old thread on the same issue https://www.askmehelpdesk.com/plumbi...-386470-4.html The only way for the tank to lose water is a less than perfect seating of the flapper. Could be junk on the rim which usually can be rubbed off with a thumb nail but it could be that your chain does not have any slack and is holding the flapper just a hair above sealing. Now the shower, I assume everything else still has good pressure and if so we need to look at junk in the mixing valve cartridge. Flow bad for bot hot and cold?