You should fill the tank up to the 'max fill line'. On my Fluval 125 it is on the left hand side.
I don't understand what you mean about the heater though...
hmmm, didnt see a max water line on the tank itself. Will check this evening.
The rim of the lid protudes about about an inch to the outside and inside of the tank glass. Therefore, due to the heater being attached very close to the glass, the top of the heater has to sit underneath the lowest part of the lid.
......should really have taken a picture for clarity.
is your water level underneath the lowest part of the lid?


