You guys using Toto's look in your tanks and inspect that flapper, check it for how it feels, they puff up and look like bubble gum. Not picking on the Toilet, just helping to avoid a maintenance issue.
Let's go through a bit of tuning procedure on a toilet if you want.
The bowl is a relatively trouble free item, the tank has all of the components within it.
If we look at the picture of the above Briggs bowl we see clearly all of the parts within.
We have a fill valve on the left, the water connects to this, this model uses a ball, you adjust a ball float by bending that rod, this one is approx 1 1/2" low. You can install a nice Fluid master fill valve in about 15 minutes.
Next we see the flush vale, that is the white thing in the middle that the flapper seats onto, often the plastic seat surface gets worn dirty or degrades, that whole item can be changed fairly easily if necessary.
Then we see the flapper, the flapper will live about 5 years unless you use products which freshen your flush in which case it lasts 2 or less. In many cases a flapper needs a float to help keep it elevated and give the bowl proper water, this float is like a small foam bobber, it gets placed just above the flapper body on the chain to give maximus flush. For conservation it gets moved to the center, this weakens the flush dwell.
Next we have the rim wash tube or bowl fill tube it needs to securely fit into the hole in the center of the flush valve.
Then we have the flush handle, it needs to be secured firmly without cracking our toilet yet needs to remain secure and not hindered in it's motion.
Those bolts in the bottom are specialty brass bolts that must use a washer between the tank and the bolts, no metal must touch the ceramic or it will crack.
Under that we have a tank to bowl washer that seals the two so the water stays off the floor and a kit to keep the tank and bowl from making contact and cracking either or both. Almost all toilets use this design. It's about 90 years old.