In troubleshooting a problem with recurring flushing (turned out to be a worn out flapper) I lowered the tank water level to about an inch below what the manufacturer marked as the waterline. Flushing seems to work fine - plenty of force, refills, fine and the flapper is fixed. I'd like to conserve water by leaving the water line where it is now, but will I be causing some kind of problem if I do?
As a volume reducer, people have put two or three bricks in the tank to conserve on the quantity of water used per flush.
Being down by 1 inch of head on the water level isn't much to worry about.
As long as it flushes the contents either way, you're fine.