What model of toilet is it? Only the C250 has low water indication, the others only have cassette full indicators.
Also, do NOT use calgon or any other descaler in any of the tanks, they will destroy the seals.
The level indicators work by a magnet inside the tanks wich affect a reed switch outside the tank.
If you have the C250 toilet and the low water light is on then the sender in the tank is faulty and it requires a new tank, or possibly a new PCB.