Two-year-old Billy Griffin wandered from his family's home in Chickerell, near Weymouth, Dorset unnoticed.
He peered over the edge of the pond and slipped into the water.
The tragedy highlights once again the dangers of ponds in family gardens. Last June, three-year-old Rodney Lawrence drowned while feeding the goldfish at the family home in Kingsclear, Hampshire.
Yesterday, an inquest was told that Billy slipped away while his mother was greeting friends who had come to visit.
He walked through the garden gate accidentally left open by one of his siblings and made his way to the pond to look at the fish – something he'd done many times before with his mum, Sarah.
Mrs Griffin realised her son was missing shortly after and desperately searched for him.
She even looked in the pond where he fell, but did not spot him because lily pads were hiding his body from immediate view.
The police were called and an officer found Billy's drowned body 30 minutes later after seeing one of his feet protruding from the water of the steep-sided ornamental carp pond.
Despite attempts to revive him, Billy was pronounced dead on arrival at hospital. Distraught Mrs Griffin later told a police officer: "I'm kicking myself now - if only I had looked."
Michael Johnston, coroner for West Dorset, recorded a verdict of accidental death at the Dorchester inquest. "When his mother was greeting guests and was distracted, somehow Billy must have got out of the house during that time," he said.
"They have a long garden and normally the gate is kept shut but on this occasion it wasn't.
"Billy had a game where he would run down the garden and his older siblings and parents would run after him and grab him and it was fun.
"This is supposition, but Billy went off and wandered into the garden and went over to look in the pond and probably stood on the parapet around it which is not very high but high enough to make it difficult to get out.
"He must have fallen in and drowned."Poor Mr and Mrs Griffin are probably torturing themselves that they let him escape - but little children do these things and I send sincere sympathy to them."