Here is the corrected version of your paragraph with proper grammar:
Herons prefer to hunt in shallow water. They stand motionless, looking out for a frog, a fish, or a bug. When they find their lunch, herons approach slowly and snatch it quickly with their beak.
Explanation:
- "In the shallow water" changed to "in shallow water" because no specific water is mentioned.
- Added a comma after "motionless" for clarity.
- "Looking out a frog, a fish or a bug" changed to "looking out for a frog, a fish, or a bug" to include the missing preposition "for" and the Oxford comma before "or bug".
- "When the lunch is found" changed to "When they find their lunch" to use active voice, which is easier to read.
- "Approach it slowly" changed to "approach slowly" because "it" is unnecessary here.