The water tank needs to be cleaned time after time and to do that you need to remove the lid, so you can thoroughly wash the tank box, float and lid itself.
See photos and follow instructions to remove the lid.
First you need to take out the float. Make sure there is no water in the tank and turn it upside down, and protect the float from falling out! 
When you have taken the float out, place your hand in the float hole, holding the longer side of the garden towards you. Gently add more pressure to lift the lid. 
It takes some strength to remove the lid. The lid itself is indeed strong, but make sure to add strength smoothly, so you won’t cause any harm to your lid.