Hi jtheuma.
To check the water level in a tank this reliable method is used, see the figure below :
Front elevation
Side view
The
item show in blue is the tank, then a elbow is made at about 5cm from
the bottom. To the elbow one connects a transparent pipe, where inside
one can put a float for better visibility of the actual level in the
tank. The level of the water in the transparent pipe will be exactly
the same as the water level in the main tank.
Note that the
pipe should be open to atmosphere at the top, and that the float is not
made tight as it can get stuck giving incorrect reading.