"I do not think [the boy's mother] can be criticised in any way for turning away for a short period of time to attend the needs of her younger daughter," he said.
"All young children need a lot of supervision and Jahmaine needed more than most but there is no evidence to suggest he did not get that degree of supervision."
Mr Scott warned of the danger of tanks and their appeal to young children.
"It is probably too much to ask that owners destroy and replace them but at the very least owners need to be aware of the problem and modify tanks in some way so that it becomes impossible for a small child to remove the manhole cover," he said.
Jahmaine's autism meant he was very active and, according to his father, had no real concept of fear.
He required a high level of supervision and had a history of running off, which his parents were well aware of. Mr Scott said despite this, Jahmaine's parents could not be expected to watch their son "24 seven, like hawkes".
Following the incident, a police investigation found the tank was at least 10 years old and not common in the area.