Not sure what observations for first part (dead plants?) But anyway for second part u can see the organism on the side on the container blocking the light. This will affect the plants in photosynthesis and hence their growth.

(b) The aquatic plants could be reaching out towards the part of the tank where organism X is absent so as to get as much sunlight as possible
(c) The growth of the aquatic plants were affected as organism X which grew on the surfaces blocked sunlight from reaching the plants, hence with lesser sunlight, the rate of photosynthesis will decrease which in turn decreases their rate of growth.